Building a Strong Test Strategy for System Migrations

By Mon Luong, Test Lead at Groove Technology

System migrations are always a challenge, and anyone who's been involved in them knows that the risks are high. It's like moving into a new house: the space might look bigger and better, but without a thorough inspection of the plumbing, wiring, and foundations, things can fall apart when you least expect it. This is exactly what happens during a system migration if you don't carefully plan your testing strategy.

In one of my recent projects, I was responsible for testing the migration of a large-scale system from .NET Framework to .NET 8, which was not without its hurdles. Unlike traditional software development, system migrations involve complex interdependencies, both within the software and across external integrations. If not tested thoroughly, the migration can lead to data corruption, unaddressed performance bottlenecks, and lost business-critical functionality. That's why a comprehensive test strategy is essential—without it, these risks can snowball quickly and compromise the entire system's stability.

In this blog, I'll dive deep into the lessons I learned from this migration, share best practices for creating a strong test strategy, and explain how a strategic approach to testing can ensure a smooth transition with minimal impact on users.

Watch my video below, where I share how we tackled a .NET migration testing challenge at Groove Technology

01. The Hidden Risks in System Migrations That Can Break Everything

Migration projects carry unique risks that aren’t always obvious at first. Unlike traditional software development, where we can control the code we build, a migration forces us to work with legacy code, outdated dependencies, and unknown system behaviors. Here are the biggest challenges we encountered:

1.1 Undocumented Legacy Features That Can Disappear Without a Trace

A surprising number of core system functionalities in older applications lack proper documentation. During our migration, we discovered business-critical logic embedded deep in legacy code that no one was aware of. If we hadn’t proactively identified and tested these, they could have vanished unnoticed, leading to incorrect calculations and broken workflows.

1.2 Integration Failures That Surface Only After Deployment

A migration isn’t just about moving code; it affects how the system interacts with databases, APIs, and third-party services. In our case, the transition to .NET 8 introduced authentication issues because encryption standards had changed. The result? Users were locked out of the system, and external integrations failed without warning.

1.3 Performance Bottlenecks That Emerge in the New Environment

Moving to a new framework doesn’t automatically mean better performance. After migrating, we noticed that API response times had increased from 300ms to over 5 seconds in some areas. The reason? Certain database queries, which worked efficiently in the old system, weren’t optimized for .NET 8’s execution model, leading to major slowdowns.

These challenges made it clear that testing wasn’t just about validating functionality—it was about ensuring the system could handle real-world use cases without breaking.

02. Why Testing a Migration Is More Than Just Checking for Bugs

A strong test strategy isn’t just about writing test cases—it’s about anticipating risks, verifying system behavior, and ensuring business continuity. Here’s how we approached testing in this migration:

2.1 Functional Testing Focused on Real Business Use Cases

Instead of relying solely on outdated documentation, we collaborated with stakeholders, developers, and end-users to map out critical workflows. This ensured that our testing covered actual business needs, not just theoretical requirements.

For example, we discovered that the reporting module had specific data validation logic that wasn’t documented. Without testing against real-world data, we could have introduced critical errors in financial reports.

2.2 Regression and Integration Testing to Avoid Broken Dependencies

Since the migration impacted multiple external systems, we conducted extensive regression testing to ensure that APIs, authentication flows, and third-party services still worked as expected.

One technique that proved invaluable was contract testing, which allowed us to verify that our APIs still adhered to predefined data exchange formats. This prevented silent failures in external integrations post-migration.

2.3 Performance Testing to Catch Issues Before They Reach Production

Performance degradation is a common issue in migrations. To prevent this, we ran load tests simulating thousands of concurrent users. This allowed us to pinpoint slow queries and optimize indexing strategies before deployment.

For example, after optimizing database calls, we reduced API response times from 5 seconds back to under 400ms, ensuring a smooth user experience.

03. Lessons I Learned the Hard Way About System Migrations

3.1 Data Integrity Must Be Verified in Multiple Stages

One of the biggest mistakes I’ve seen in system migrations is assuming that data will transfer perfectly from one system to another. In reality, even a tiny mismatch in field mappings can cause massive errors in reporting, analytics, and transactional records.

During our migration, I learned that data validation must be done at three key stages:

• Before migration: We ran data profiling to identify inconsistencies in the legacy database.
• During migration: We used real-time validation scripts to flag records that weren’t mapping correctly.
• After migration: We built reconciliation reports to compare old and new datasets, making sure no data was lost or altered.

Skipping even one of these steps would have led to missing records, inaccurate financial reports, and major operational disruptions.

3.2 Expect the Unexpected and Build Contingency Plans

I remember a moment during this project when we thought we had tested everything—until users started reporting that some of their automated workflows weren’t triggering. It turned out that a third-party service we relied on changed its API format after we migrated.

What I took away from this is that no migration is ever truly predictable. Now, I always build contingency plans into our test strategy, including:

• Keeping the old system running in parallel for a short period after deployment.
• Implementing rollback plans in case critical issues emerge.
• Having dedicated post-migration monitoring teams to catch problems before users do.

That experience reinforced that even with a solid test strategy, migrations require flexibility and adaptability.

3.3 The Best Test Strategies Include Exploratory Testing

Automated test scripts are essential, but they only check what they’re programmed to check. They don’t catch unexpected interactions, usability issues, or undocumented behaviors.

One of my testers found a critical issue purely by accident—while stress-testing the new system, they noticed that exporting large reports would crash the server. Our automated tests hadn’t flagged this because they only checked export functionality at a basic level.

Now, I ensure that exploratory testing is part of every migration project. It allows testers to think like end-users and spot gaps that automation alone can’t cover.

04. My Advice for Anyone Testing a System Migration

4.1 Treat the Test Environment Like a Live System from Day One

If you wait until the last minute to conduct realistic testing, you’re setting yourself up for failure. I always insist on mirroring production conditions as early as possible—this means using real data, real workloads, and real integrations.

The more accurately your test environment reflects the live system, the fewer surprises you’ll face post-migration.

4.2 Testing Is Not Just for Testers—Involve the Entire Team

One mistake I’ve seen repeatedly is teams treating testing as something only QAs are responsible for. Developers, business analysts, and end-users all bring critical perspectives that testers might miss.

In our project, a business analyst flagged an issue with a complex approval workflow that wasn’t behaving correctly post-migration. Without their input, we wouldn’t have caught this before go-live. Now, I always ensure that stakeholder involvement is part of our testing strategy.

4.3 Document Everything—It Will Save You Later

The number of decisions made during a migration is staggering—what to update, what to deprecate, what workarounds to apply. If these aren’t documented, future teams will be left guessing.

I maintain a central migration knowledge base in Confluence, logging every issue, resolution, and test case. This has become invaluable when troubleshooting production issues months after migration.

05. Conclusion: A Flawless Migration Begins with Testing, Not Luck

Looking back, I can confidently say that a strong test strategy was the key to making this migration successful. It wasn’t just about finding bugs—it was about ensuring business continuity, minimizing disruptions, and optimizing system performance.

System migrations are always unpredictable, but with proactive risk management, real-world test scenarios, and cross-team collaboration, you can significantly reduce surprises and ensure a smooth transition.

If you’re tackling a system migration and want to discuss best practices for building a robust test strategy, feel free to reach out at [email protected]

Mon Luong is a Test Lead at Groove Technology, specializing in software quality assurance, system migration strategies, and performance optimization.