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.
Right, the services are different enough in implementation that any sort of “seamless” thing would be a huge lift.
Better to just have a new app (Artemis, Apollo’s twin?) integrate with Lemmy/Fediverse stuff with the same “feel” as the original.