Lemmy could really do with the ability to transfer an account to a new instance. The only reason I'm still on lemmy.world is because I can't be bothered to set up an account on another instance that may be as insufferable.
As for communities, Lemmy just isn't large enough to be picky. World is the biggest instance, but is still dead on anything that's not either a Reddit clone or a popular community.
There is an export feature that will spit out a json file and you can import it to a new instance. It will carry over your subbed communities and blocked accounts. I use it to sync accounts on different instances (My instance can be slow sometimes)