[This is preliminary documentation and subject to change.]
The migration DLL is a standard Win32®-based DLL you create. It is called by Windows NT 5.0 Setup at different times during the upgrade process to perform a custom migration of your otherwise incompatible application. Setup relies on migration DLLs to identify unknown software components and to either migrate their settings to Windows NT or remove the components that will never work properly after an upgrade.
At a minimum, a migration DLL must have the following Setup-defined exported functions.
Your migration DLL can contain additional functions. But if any of these seven functions is missing from a migration DLL, Setup will not load and process it, and your application will not migrate.