I mean it’s easy to reimplement entire games if you’ve built it modularly. Just swap your core game logic to run on another library and the game works the same it did before.
Edit: 'course, exceptions exist like if you wrote everything using their proprietary coding language, instead of using something universal.
Edit 2: It MAY still be possible that a translation/compiler exists that’ll run as a plugin in a proprietary engine, and converts it into something universal.