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’m using Mlem as well. I’m interested in trying any apps to see what is out there. Right now this has a really nice feel about it. Definitely enough to get me started and more useful than the web app on my iPhone.