How We Modernized a Legacy Application With a .NET 8 to .NET 10 Migration

Quick Summary

Introduction

Most .NET applications reach a stage where keeping them on an older framework costs more than moving them forward. Legacy application modernization is how teams get ahead of that, and for anything still on .NET 8, the clearest path is a .NET 8 to .NET 10 migration. .NET 8 is a solid framework and still runs well in production. The catch is its support window. Once the patches stop, every open vulnerability simply sits there, unfixed.

.NET 10 is where that pressure gets resolved. .NET 10 is where that pressure gets resolved. It is a Long Term Support (LTS) release, so it gets three years of patches. It also runs faster and handles cloud and container workloads better than .NET 8 did. Whether you upgrade now or later usually comes down to money. Plan the .NET 8 to .NET 10 migration properly and it costs a fraction of what you would spend scrambling through it after support has ended. We did this upgrade recently for a client whose main revenue app was still on .NET 8. Below, we cover how it went, the parts that tripped us up, and the breaking changes you will want to know about before you begin.

Why the Client Needed a .NET 8 to .NET 10 Migration

Legacy systems rarely fail all at once; more often, performance degrades gradually until the impact becomes difficult to ignore. In our client’s case, response times had steadily increased over time, while an aging dependency stack made the system increasingly costly and complex to maintain. The gap between their framework and current tooling had widened so far that the team spent most of its time just keeping the application running instead of improving it. For a codebase in that shape, a .NET 8 to .NET 10 migration is usually the most useful single move you can make, because it deals with performance, security, and maintainability in one go instead of one at a time. Fixing all three in a single planned effort, rather than chipping away at them separately, is exactly what our .NET migration services are built for.

The cost angle is hard to ignore, as maintaining legacy systems can consume up to 70% of IT budgets in some sectors. Much of this spending goes toward upkeep rather than delivering any new features. Waiting only makes the situation worse, as each skipped release increases the overall workload. It also widens the gap you will need to close later.

Security mattered even more, because once a runtime loses support, it no longer receives patches. Attackers are well aware of this gap and actively exploit it. Older financial platforms reportedly face around three times as many cyberattacks as current ones. Our client’s application handled payments and personal data, so staying on an unsupported framework was a compliance problem, not just a technical one. That is why we treated this legacy application modernization as a requirement rather than a nice-to-have. Upgrading before support ends also keeps the timing in your hands. You schedule it around the business instead of scrambling when a deadline forces the issue.

What Made .NET 10 the Right Upgrade Target

We do not jump on every new release, so we weighed the target before committing. .NET 10 won out mainly because it is an LTS version. It ships with three years of support, which gave the client a stable, patched base and room to plan the next upgrade on their own schedule rather than in a hurry. That alone lowered the risk of the whole project.

The performance gains are real, and they show up in production, not just in benchmarks. Microsoft reports that changes to the Arm64 write-barrier cut garbage collection pause times by 8 to 20%, on top of JIT compiler work in inlining and devirtualization and wider hardware acceleration through AVX10.2. If an application is held back by GC pauses or heavy serialization, this is where it benefits most, and none of it requires rewriting your own code.

The client runs everything in containers, so the improvements to Native AOT and trimming had a direct payoff: smaller images and faster cold starts, which mean lower hosting bills and quicker autoscaling. Put together, these were the reasons we went straight to .NET 10 for the .NET 8 to .NET 10 migration instead of stopping at an interim version. If you are deciding which version to upgrade to, our Guide to Upgrading .NET Framework for 2026 outlines the complete path across recent releases. For any meaningful legacy application modernization effort, .NET 10 is a practical and forward-looking choice.

There is more to it than speed, as .NET 10 streamlines many smaller aspects across ASP.NET Core, Entity Framework Core, and the base libraries. This reduces the need for custom workarounds that tend to accumulate in older codebases over time. For the people running the team, that adds up to less maintenance down the line, and in our experience, that weighs on the .NET 8 to .NET 10 migration decision just as much as the performance numbers.

Our Step-by-Step .NET 8 to .NET 10 Migration Approach

We handled the .NET 8 to .NET 10 migration as a fixed sequence, not a pile of edits made in whatever order came to mind. The one rule we stuck to was simple: update the dependencies before touching the target framework. That order cleared out most of the breaking changes before the compiler ever saw them. Before starting the upgrade, we created a separate Git branch for the work and confirmed that all existing tests passed on it. This gave us two things: a baseline of how the application behaved on .NET 8, which we could compare the upgraded version against, and a safe point we could return to if anything went wrong. The following is the process we followed, step by step.

Step 1: Assess the Codebase and Set Up the .NET 10 SDK

We installed the .NET 10 SDK alongside the existing one, so both runtimes were available at once, and updated the build toolchain to match. Then we ran dotnet list package outdated to list every dependency and flag anything with no .NET 10-compatible version. We went through each project’s target framework, picked out packages that were already deprecated or no longer maintained, and made a note wherever the code touched an API that .NET 10 had changed. That gave us the true size of the job and showed us the riskiest dependencies before we changed a single line. On a big solution, this is the step that decides whether the work takes two weeks or two months, because the nasty surprises almost always come from a dependency nobody remembered was still in there.

Step 2: Update NuGet Packages Before Switching the Framework

With the project still on net8.0, we brought the Microsoft and third-party packages up to their 10.x versions and kept an eye on the build to make sure it stayed clean. The solution used central package management through a Directory.Packages.props file, so we set the versions in one place and every project picked them up. Transitive dependencies needed attention as well, not just the ones we referenced directly, because an outdated package sitting two levels down can hold up the whole upgrade. We ran the tests again after each round of updates. Sorting out the packages first, while we were still on a framework we knew worked, kept the dependency problems from getting mixed up with the framework ones. When something did break, we knew straight away which change had caused it.

Step 3: Change the Target Framework and Rebuild

Next we switched the target framework moniker from net8.0 to net10.0, pointed global.json at the new SDK, and ran a clean restore and rebuild. In a multi-project solution, we started with the shared libraries at the bottom of the dependency chain and worked up toward the web and API projects, so each layer built on one that already compiled. The first rebuild threw the expected wave of errors and obsolete-API warnings. Rather than patch them at random, we grouped them by type and worked through one category at a time. This is the step that revealed the framework-level changes needing real code fixes, as opposed to package bumps.

Step 4: Fix Breaking Changes and Deprecated APIs

Then we worked through the changes .NET 10 brought in. That meant switching to AddAuthorizationBuilder(), dropping the old UseEndpoints() registrations, and adjusting for stricter JSON serialization, where types that used to deserialize loosely now needed explicit handling. The built-in analyzers and obsolete warnings did a lot of the finding for us, pointing straight at the lines that needed attention. Configuration binding and any custom middleware got a second look too, since those often depend on behavior that changes without much warning from one version to the next. We checked every fix against Microsoft’s official .NET 10 migration guidance instead of guessing, and we committed in small, clearly labeled batches. That kept the history readable and made any single change simple to undo if we needed to.

Step 5: Test, Update CI/CD, and Deploy

Last came the testing and the deployment. We ran the full test suite, switched the Docker base images and the CI/CD pipeline to the .NET 10 runtime, and rolled the release out gradually. One thing we were careful about was updating the build agents and the container images together, so the pipeline and the runtime stayed on the same version. We did not cut everything over in one go. A few percent of live traffic went to the new build first, and we watched how it did on error rates, response times, and memory next to the old build. The thresholds for rolling back were set before any of this began, so if the new build slipped, the call was already made for us. It did not slip. The numbers stayed clean, and we opened it up to everyone.

The Migration Challenges We Ran Into (and Fixed)

Even a well-planned .NET 8 to .NET 10 migration hits snags, and ours was no different. Knowing where the trouble tends to show up is what turns a production incident into a task you have already scheduled.

Third-Party Library and NuGet Compatibility

The most common blocker is a dependency that has no .NET 10 release yet, and we ran into it. Two libraries in the client’s codebase had nothing compatible when we started. One had a newer version we could move to, though it came with its own API changes to sort out. The other we swapped for a maintained alternative and reworked the code that called it. We caught both during the assessment, so neither turned into a last-minute scramble.

Breaking Changes in Authorization and Routing

The authorization and routing changes needed careful review, not a blind find-and-replace. We went through each policy registration and checked the order of the middleware to keep the behavior the same, then covered those paths with regression tests before we called the change done.

Configuration and Dependency Injection Conflicts

A few service registrations had leaned on the older runtime’s looser behavior, and they broke under .NET 10’s stricter validation. We rewrote them and confirmed that startup validation passed cleanly. Catching this early mattered, because these are the kind of quiet bugs that are difficult to track down once they reach production.

How We Tested and Deployed the Migrated Application

Testing is where the .NET 8 to .NET 10 migration proved itself, and a clean compile was never enough for us. We ran the client’s existing unit and integration tests first to set a baseline, then wrote new tests around the parts we had changed, mainly authorization and serialization, to confirm the important paths still worked from start to finish.

Benchmarking was just as important, so we ran the same workloads across both runtimes and measured response time, throughput, and memory usage. That gave us real numbers to point to instead of a gut feeling, and it let the client’s leadership see for themselves what the upgrade was worth. The deployment got the same care. It went to staging first for smoke and load tests, then to production behind a monitored rollout with a rollback standing by. We never reached for it. Handling a release this way keeps a .NET 8 to .NET 10 migration low-risk even on a busy system the business runs on, and the before-and-after figures doubled as a clean compliance record for the client.

Results After the .NET 8 to .NET 10 Migration

The .NET 8 to .NET 10 migration delivered on the reasons the client took on this legacy application modernization in the first place. The results showed up across performance, cost, and security:

• Faster response times: Average API response time under peak load dropped from about 480ms to 210ms, a 56% improvement, and 95th-percentile latency fell from roughly 1.2s to 400ms. The garbage collection pauses that used to spike during busy periods mostly went away.
• Less hardware for the same work: The application handled the same traffic on 3 container instances instead of 5, cutting compute usage by around 35%.
• Lower cloud costs: Smaller images and lower memory use brought the monthly cloud bill down by roughly 22%.
• Quicker maintenance and onboarding: Average patch-and-release time shortened from about two days to half a day, and new engineers reached their first merged pull request in around a week instead of two to three.
• Stronger security and support: The application moved from an end-of-life runtime to a supported LTS version with patches guaranteed through 2028, closing 100% of the exposure tied to running unsupported software.

Conclusion

Frequently Asked Questions (FAQs)

How long does a .NET 8 to .NET 10 migration take?

It depends on how big the codebase is and how tangled its dependencies are. A small, tidy application can move in one to two weeks. A large system with a lot of third-party libraries can take several. A proper assessment up front is what keeps the estimate honest.

Is a .NET 8 to .NET 10 migration worth it for a stable application?

Yes. .NET 8 reaches end of support on November 10, 2026, and after that there are no more security patches. Aside from removing that risk, .NET 10 brings real performance and hosting-cost improvements, so you keep getting value from the upgrade rather than just holding steady.

What is the most difficult part of moving to .NET 10?

The hardest parts of a .NET 8 to .NET 10 migration are usually third-party library compatibility and the breaking changes in authorization and routing. Both are manageable if you audit the dependencies first and cover the changed areas with tests before you deploy.

Should a team move directly to .NET 10 or upgrade incrementally?

For most teams, going straight to .NET 10 is the better call. It is an LTS release with support through 2028, so a single .NET 8 to .NET 10 migration saves you from adopting an interim version and then having to leave it behind soon after.