Oooh, okay. So if I subscribe to !startrek@startrek.website, then kbin would store that data, and I would be able to point everyone to !startrek@kbin.social and we'd all be able to pick up where we left off? Still an issue of getting users to change where they're posting to, but that's better than I thought, at least.
Also, I imagine the problem with how difficult it is to migrate away from commercial centralized services is that it's hard to spin up a new version of that site with the code and database. Being quick to spin up a new kbin instance or Lemmy instance helps immensely, though the issue of directing the users to those new instances would be just as difficult.