There is one guy running the lemmy.world instance i believe, I can't seem to get the correct information about it straight in my head so I'll just post a link to his profile to learn more about him, he makes a lot of posts relevant to the running of the server.

https://lemmy.world/u/ruud

The fact the PC version of Lemmy is very similar to old.reddit made the switch a ton easier for me, jerboa was pretty good although it's a bit broken for .world currently, and the other app i tried (connect) is... alright, not a fan really, but it does work so it gets the temporary job. I used a bit of reddit, but nearly only on phone and that's gone so i guess im done with it. I keep opening it with muscle memory a few times a day and then realise im not meant to be doing that, hopefully that goes away.

The previous posts would remain, but new posts on that instance shouldn't show up for anyone else, although i did notice some beehaw posts on my subs list after they defederated (that one wasnt actually anyone being a dick or getting screwed over, they had concerns about a couple instances having open sign ups) so that might not entirely work, or i may have misunderstood it. I know and have seen that comments won't show up after the defederation though.

You might notice a few different instances have the same or similar communities, i expect they would just migrate over and because it's still Lemmy there should be basically no learning curve. Not great, but significantly better than if you had to start from scratch on a new website.