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.
I mean, we all had to log into the reddit apps in the first place anyways, so I don’t see why we can’t just do that again instead of trying to guess instances or something