FYI this is from 2022 but is relevant because Thunderbird (and K9 v8) just landed in F-Droid (and other android package managers I guess but I haven't checked).
The process of importing from K9 Mail worked without issue for me. There is also the option to import from desktop using a QR scanner (or some kind of scanner, again I didn't go down that rabbit hole).
How did the K9 project never talked about this? Its public since 2 years or so that Thunderbird announced they are working on rebranding K9 as Thunderbird.
Edit: My bad English. I tried to rephrase this reply.
But the blog post is 2 years old and not part of the application. Since then new users started using the app. Most people don't read blog posts, if nothing big changes. Was there never a popup message, in example after a regular update, to inform about upcoming huge changes? So that people do not get surprised. The app description should have this information very clear and prominently on the top. No current user should be in a position that the app changes like this.
Ah, yeah, I don't think there was anything in the app. I guess, they could've mentioned it in the changelog, which gets shown in the app by default after an update.
But yeah, I think we'll have to excuse a bit of a bumpy ride here. I know, it says "Mozilla" on there now, but to my knowledge, it's still just the one core dev...
In F-Droid (not sure about Play or other app stores) K9 v8.0 (and above I guess) is now listed as Thunderbird Beta for Testers. I and possibly others thought that the K9 version of the app would keep it's branding all the way through, including it's listing name or title in various app stores. Perhaps it wont or perhaps it's a listing error. We'll see.