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How to Plan a Successful Migration From WebForms to .NET Core in 2025

Published on 3 Nov 2025 by New Media Aid — bespoke SME app development since the year 2000

WebForms .NET Core Modernisation SME Software Migration Razor Pages Legacy Systems

Upgrading a long-running ASP.NET WebForms system to .NET Core can feel intimidating, especially for SMEs who rely on their system daily. But with the right plan, you can modernise safely, avoid downtime and dramatically improve performance and maintainability.

This guide outlines how to plan a realistic and successful migration in 2025.

Why a WebForms → .NET Core Migration Is Worth It

WebForms is no longer being developed, and its ecosystem is ageing rapidly. Migrating brings several benefits:

  • Modern, responsive UI with Razor Pages
  • Better performance (no ViewState, async-first, lighter pages)
  • Lower hosting costs with Linux or container options
  • Cleaner architecture with dependency injection
  • Fewer bugs and easier maintenance
  • Improved security with modern authentication and middleware

Most SMEs see improvements within the first few weeks of rollout.

The #1 Rule of Successful WebForms Migration

Don’t rebuild everything at once.

Big-bang rewrites are expensive, risky and often fail. The best approach is incremental migration: build the new .NET Core system alongside the old one and replace modules one by one.

The Migration Roadmap

This phased approach keeps your team productive and reduces risk.

1. Audit the Existing System

Start by identifying:

  • All pages, modules and workflows
  • Which parts of the system are still used daily
  • Which modules are buggy or hard to maintain
  • Database structure and dependencies
  • Hard-coded logic inside WebForms code-behind files

This gives you a clear view of what you’re migrating.

2. Identify “High Pain” and “High Value” Areas

These modules should be migrated first:

  • Slow pages
  • Buggy workflows
  • Frequent support requests
  • Critical business processes

You’ll often find 20% of the system causes 80% of the pain.

3. Create a Modern Architecture Blueprint

Before writing code, design how the new system will work:

  • Use Razor Pages or ASP.NET MVC (Razor Pages is perfect for SME apps)
  • Introduce services for business logic
  • Use Dapper or EF Core for clean data access
  • Implement dependency injection
  • Set up middleware for logging and authentication
  • Adopt modern CSS frameworks like Bootstrap 5

This ensures the new app is fast, testable and future-proof.

4. Migrate the Database Layer First

To avoid breaking WebForms, keep the existing SQL Server database but:

  • Build a clean data-access layer in .NET Core
  • Introduce stored procedures or services where appropriate
  • Optimise slow queries and add missing indexes
  • Refactor SQL before refactoring UI

This gives you a solid foundation for the new system.

5. Build New Modules One at a Time

Start with small, self-contained modules such as:

  • Dashboards
  • Search pages
  • Basic CRUD screens

Then move on to larger, workflow-heavy modules like:

  • Jobs
  • Quotes
  • Scheduling
  • Reporting

Each module can run on .NET Core while others remain on WebForms.

6. Use a Shared Login Between Old and New Systems

This allows staff to move between the WebForms and .NET Core areas seamlessly. Options include:

  • ASP.NET Core Identity
  • JWT tokens
  • Shared cookie-based auth

Choose the simplest option that works for your setup.

7. Test With Real Data, Not Sample Data

Migrations often fail because test systems don’t reflect real-world usage. Make sure you test with:

  • Large data sets
  • Old historical records
  • Real user behaviour
  • Edge-case workflows

This avoids surprises when you go live.

How Long Does a Migration Take?

For SMEs, typical timelines are:

  • Small systems: 2–3 months
  • Medium systems: 4–7 months
  • Large systems: 8–18 months (using phased rollout)

Most SMEs choose a part-time, steady migration while keeping their current system fully operational.

Cost Considerations

Migrating to .NET Core is an investment, but it quickly pays off by reducing:

  • Developer costs
  • Server costs
  • Bug-fixing time
  • Business downtime
  • Staff frustration

The performance and productivity improvements are often dramatic.

Why Migrating in 2025 Makes Sense

  • WebForms ecosystem is shrinking rapidly
  • Browser support for old JavaScript dependencies is weakening
  • Modern APIs (e.g., OpenAI, payments, maps) integrate better with .NET Core
  • Security expectations are far higher than in 2010
  • Hiring developers familiar with WebForms is increasingly difficult

Delaying migration only increases risk and cost.

How I Help SMEs Migrate Safely

I specialise in modernising WebForms systems without disrupting business operations. My process typically includes:

  • Full system audit and migration plan
  • Incremental rollout with zero downtime
  • Clean architecture and modular design
  • SQL performance tuning
  • Modern UI/UX improvements
  • Robust authentication and security upgrades

The aim is a smooth transition — not a risky rewrite.

Thinking About a WebForms Migration?

If you’d like a realistic, tailored roadmap for modernising your WebForms system, I’m happy to help.

Email: ab@newma.co.uk
Phone: +44 7967 219288

No obligation — just clear technical guidance based on 25 years of experience.