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.
These apps seem pretty similar (although new and barebones): memmy https://testflight.apple.com/join/6jaRU6rD
Mlem https://testflight.apple.com/join/xQfmkJhc
Using Mlem currently, think it has a ton of potential! Though I will say it crashed as I tried to reply the first time lol. Has a lot of crashing issues, but if they can get it stable and make steady improvements it’s got excellent potential.
Memmy is also good. It lets you swipe to upvote like Apollo!