The illusion of stability is the worst enemy of any IT Director. Many companies live for years under the assumption that “if it works, don’t touch it,” but fail to realize that their technological foundation is slowly turning into quicksand. Legacy .NET Framework is no longer “the previous version of the platform,” but rather a direct, tangible business risk that grows at a geometric rate. With every day of downtime, the cost of infrastructure ownership grows, and finding specialists to work on the code from the previous decade becomes a real quest.
Phase 1: Assess & Prioritize
Any large migration effort undertaken without a thorough knowledge of the landscape is doomed to suffer from perpetual budget blow-ups and missed schedules. Dependency chaos, missing documentation, and “magical” bits of code that no one dares to touch require a cold, analytical approach. The process must begin with a deep audit where you clearly understand what you are dealing with, from third-party libraries without source code to hard bindings to the system registry. Expert teams like Softellar always start with a full inventory of digital assets to eliminate unpleasant surprises at the production stage.
Inventory & Categorize
You need to divide your application portfolio into categories to understand the real scope of work. Not every service requires immediate rewriting. Use specialized tools like .NET Portability Analyzer or try-convert to automatically check the compatibility of binaries and source code. The main blocking factors are often hidden in architectural details:
- Strong dependence on ASP.NET Web Forms, a technology that has no direct migration path to .NET 8 and requires rewriting the UI;
- Use of WCF for interservice communication, which often requires replacement with gRPC or REST API;
- Outdated NuGet packages that are no longer supported by their authors and have no versions targeting .NET Standard;
- Direct Win32 API calls or dependence on COM components, which make containerization on Linux impossible.
All of it will allow you to quickly filter out modules ready for migration with minimal adjustments and identify those that require serious surgical intervention. Technical debt has been accumulating for years, and trying to repay it in a single sprint will inevitably paralyze business operations.
Build the Business Case
At this stage, it is critically important to build a well-founded business case for stakeholders. Priority should be given not to the modules that are “easiest to migrate,” but to those that carry the highest risk of failure or generate the company’s key revenue. The ROI of migration is formed from concrete numbers: savings on Windows licensing (when switching to Linux/Kestrel), reduced cloud resource costs due to performance optimization, and a radical acceleration of Time-to-Market for new features. Migration for the sake of migration benefits no one; what you need is a measurable result.
Phase 2: Choose Your Path
The form that the migration strategy takes will not only define the budget but will also define the future of the product. There is no way to apply a template to the entire landscape of corporate applications. Flexibility and pragmatism are required in this situation, as there are different advantages and risks in each situation.
Rehost (Lift-and-Shift)
This method is often called a “quick win,” but that is a debatable claim. You simply take the application as is and move it to cloud infrastructure (IaaS) or Windows containers. The code remains almost unchanged. This is suitable for stable legacy systems that do not require further development but must be removed from the on-premises data center. You keep dependencies on the old OS and the heavy runtime, but gain some flexibility in resource management. Obviously, the long-term benefits here are minimal, since you are transferring all architectural problems to a new platform.
Refactor to .NET 8 (Recommended)
The golden mean for most active corporate systems. You do not rewrite business logic from scratch, but gradually replace outdated technical components. The code is adapted to work with .NET Standard, Dependency Injection is introduced, and configuration is moved from the bulky web.config to the flexible appsettings.json. This is the best balance of cost and outcome, allowing you to start using the power of .NET 8 and Linux containers without risking a full stop of product development for a year.
Rebuild (Cloud-Native)
A complete rewrite of the application based on Cloud-Native principles and microservice architecture. This is the most expensive, lengthy, and risky path. It is justified only for mission-critical systems that have hit the scalability ceiling or require a radical change in functionality. Here, you get maximum performance, resilience, and the ability to use Serverless, but you pay for it with high demands on the team’s expertise.
A comparison table of the approaches is provided below:
| Feature | Rehost strategy | Refactor strategy | Rebuild strategy |
|---|---|---|---|
| Primary focus | Infrastructure migration | Code modernization | Architecture redesign |
| Cost efficiency | Low upfront cost | Medium investment | High initial capital |
| Time to deploy | Weeks | Months | Six months plus |
| Scalability potential | Limited by legacy | High (containers) | Maximum (microservices) |
| Risk level | Low technical risk | Moderate complexity | High operational risk |
| Skill requirement | Ops focused | Dev and ops blend | Advanced architecture |
| Long-term value | Low | High | Maximum |
A strategic vision is needed to make a decision on the path. Just like experts in their field need guides to be able to visualize their marketing objectives, technical architects need to be able to visualize the data flow before the project even starts. Otherwise, the project can easily turn into an eternal construction site without any results.
Phase 3: Execute & De-risk
At the execution stage, the primary objective is to minimize the risks for the running business. The biggest blunder is to attempt to do everything at once. A Big Bang migration, where the old system is shut down on Friday evening, and the new, ‘perfect’ one is cutover on Monday morning, is bound to fail in the real world. Modern engineering practices mean a seamless transition that is completely transparent to the end user.
Never “Big Bang”: Use the Strangler Pattern
Your best friend is the Strangler Fig pattern. The idea behind this pattern is that you gradually replace the old system’s functionality with the new microservices. You create a new microservice using .NET 8, such as an authorization service or a shopping cart service. You deploy an API Gateway in front of the system. The API Gateway will call the new microservice; the rest will be proxied to the old system. The old system will gradually die as it “strangles” away. Users will use the old system as normal without even realizing that a new system has been deployed behind the scenes.
Run in Parallel
The insurance that is your guarantee of logical errors is parallel execution (Shadowing). The new version of the service runs side-by-side with the old one and uses a duplicate of real traffic. The outcome of the new version, however, is not passed to the end user. In this manner, it will be possible to compare the work of the algorithm and follow the work of the system under load and verify bugs in real-life circumstances. When the metrics of the new system have leveled off and are comparable to those of the baseline, then you can begin to send real traffic.
Automate Relentlessly
Manual deployments and configuring servers “from memory” are unacceptable in the Cloud-Native era. You need a reliable CI/CD pipeline that automatically builds the code, packages it into containers, and runs it through thousands of tests with every commit.
Implementing full automation includes:
- Configuring unit tests and integration tests that cover critical business scenarios and regression;
- Automatic scanning of Docker images for vulnerabilities before pushing them to the registry;
- Using Infrastructure as Code (Terraform, Bicep) to manage environments and eliminate configuration drift;
- Creating automatic rollback scenarios if system health metrics go out of bounds after deployment.
Automation also reduces cognitive load on the team, freeing developers’ time for product tasks instead of fighting infrastructure. It is important to understand that .NET 8 provides excellent built-in tools for observability and metrics (OpenTelemetry), which must be implemented from the start. Without proper monitoring, you will be flying blind, learning about problems from angry users.
Conclusion: The New Default Choice
Moving to a new stack is a painful but necessary evolutionary process. A successful migration will transform the IT organization from a “cost center” into an innovation enabler. The system will become scalable, secure, and ready for integration with AI, big data, and new cloud services. Upgrading mission-critical applications is always a high-stakes game. Thus, modernizing critical legacy apps is high-stakes. Partnering with a specialist ensures the methodology and expertise to turn a challenge into an advantage.
Published: February 2, 2026
