Do they get adopted by other instances? Are they still accessible from other instances? Can you still post on them from another instance?
Edit: From my understanding every instance that deals with a community has a cached copy. Will that copy disappear after a certain time, because it can’t phone to home anymore?
Not sure how it is on lemmy. But looking at the structure on kbin. I reckon you could (with a little sql magic) convert the existing one to a local magazine without cloning, and then people could subscribe to the new version or existing subs could also hack their sql to change the id to match the new instance and toggle the subscriptions.
On Lemmy though I think images are not cached locally. So you might lose those. Kbin by default will also download images/media locally too.
Not sure this would happen enough to add formal functionality for it though.