Presumably we'll see that happen here as well, just potentially at a higher level, with instances rather than just communities.
I recently had to migrate my Mastodon account (home.social shut down, for... frankly frustrating reasons, but whatever). It was a pretty painful process, and none of my old toots exist as a result.
Hopefully we can figure this out with Lemmy, and in doing so, make migration painless enough that it can even be a common occurrence without a loss of data and content. Let's not forget that one of the great things about Reddit is (was, I guess) that old threads still contain relevant information from passionate enthusiasts.
From what I've seen so far, that doesn't seem to be a solved thing yet.