Make it seamless for people to migrate. Duplicate comments/posts/vote API calls to Lemmy. Then they could just drop the Reddit side whenever they want, and users wouldn’t have to do any work. Most wouldn’t even notice the change.
Of course they’d have to keep the app up in the meantime, but I’d donate to that fund.
Say I made a post to some specific subreddit from apollo. How does the app decide which lemmy community to use, on which instance? Or if I voted or made a comment, on which post on which community on which instance should it land?
I don’t think this is a serviceable idea.