Add WPF/WinForms upgrade-vs-rebuild decision guide to ai-modernize.md (#6657)

* Add WPF/WinForms upgrade-vs-rebuild decision guide to ai-modernize.md

Adds a 'Should I upgrade in place or rebuild with WinUI 3?' decision
table near the top of the WPF/WinForms section, covering:
- Upgrade in place (.NET 9+) via Copilot App Modernization agent
- Add Windows App SDK features without framework change (winapp CLI)
- Rebuild with WinUI 3 (Copilot migration + wpf-patterns-winui3.md)
- LOB apps with WCF/COM: upgrade-first strategy

Closes dotnet/docs-desktop#2218 (superseded by this approach — WinUI 3
rebuild path lives in Windows docs; .NET upgrade path belongs in
docs-desktop alongside issues #2165/#2166).

Co-authored-by: Copilot <[email protected]>

* Address Copilot feedback on ai-modernize.md decision table

- Fix 'two distinct paths' -> 'a few distinct paths' (table has 4 rows)
- Clarify 'Rebuild with WinUI 3' row: lead with WPF-specific API mapping,
  note the UWP migration link is UWP-specific but workflow is similar
- Soften LOB row: 'handles' -> 'can assist with' WCF/EF/ASP.NET upgrades
- Standardize terminology: 'Copilot Modernization agent' -> 'GitHub Copilot App Modernization agent'

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>

Author: GrantMeStrength

hub/apps/windows-app-sdk/migrate-to-windows-app-sdk/ai-modernize.md

@@ -2,7 +2,7 @@
 title: Modernize or port a Windows app with GitHub Copilot
 description: Use GitHub Copilot, the WinUI 3 migration plugin, and winapp CLI to migrate UWP apps to WinUI 3, modernize WPF/WinForms apps, and add Windows features to cross-platform apps.
 ms.topic: how-to
-ms.date: 03/10/2026
+ms.date: 04/17/2026
 ms.author: jken
 author: GrantMeStrength
 keywords: windows, github copilot, uwp migration, winui 3, wpf, winforms, modernization, winapp cli
@@ -72,6 +72,20 @@ Copilot handles most namespace remapping and straightforward API substitutions w
 
 ## Modernize a WPF or WinForms app
 
+### Should I upgrade in place or rebuild with WinUI 3?
+
+Before you start, it's worth deciding which direction makes sense for your app. There are a few distinct paths with different tooling and scope.
+
+| Your situation | Recommended path |
+|---|---|
+| Keep WPF or WinForms as your UI framework, move to .NET 9+ | **Upgrade in place** — use the [GitHub Copilot App Modernization agent](https://aka.ms/copilot-app-modernization) to automate the project file and code changes |
+| Add modern Windows features (notifications, AI, shell integration) to your existing app without changing the UI framework | **Add Windows App SDK features** — see the scenarios below; use winapp CLI to add package identity if needed |
+| Need modern Fluent UI, touch support, WinRT depth, or a fresh codebase | **Rebuild with WinUI 3** — see [WPF patterns in WinUI 3](wpf-patterns-winui3.md) for API mapping; for a Copilot-assisted approach see [Migrate a UWP app to WinUI 3](#migrate-a-uwp-app-to-winui-3) (UWP-specific, but the Copilot workflow is similar) |
+| LOB app with WCF, COM, or heavy third-party UI controls | **Upgrade in place first**, then modernize incrementally — the [GitHub Copilot App Modernization agent](https://aka.ms/copilot-app-modernization) can assist with WCF → CoreWCF, EF, and ASP.NET Core upgrades |
+
+> [!NOTE]
+> "Upgrade" (moving to a newer .NET version on the same UI framework) and "rebuild" (moving to WinUI 3) are separate decisions. You can upgrade now and rebuild later, or do both together for a greenfield replacement. The GitHub Copilot App Modernization agent covers the upgrade path; the sections below cover adding Windows App SDK features to your existing app.
+
 WPF and WinForms apps can adopt Windows App SDK features without a full rewrite. Copilot + the Learn MCP Server makes it easy to find and integrate specific features.
 
 ### Add push notifications