This is such an important piece of information. Imagine you opt for Next and have to rewrite your app soon. I know, APIs can be backwards compatible and Next's next version will be backwards compatible but there is always something which doesn't work, sending you down the rabbit hole...
Your apps should not be affected at all.
The parent says something different and if it were just about adding types then you need just to add a file to DefinitelyTyped which I guess happened already.
We're not even "rewriting everything in Typescript at once". We're slowly converting files over one by one. No public APIs are changed. Furthermore, we have an extensive test suite of end-to-end tests for both development and production behavior.