The thing is that the value is in the communities and not in the old content. So most likely the mods would just post we move to a new instance and a lot of users would follow. We just saw that on the German speakin lemmy instance feddit.de, which was abandoned and now most of the users and communities moved to feddit.org, which is already one of the larger ones.
What lemmy really needs is the ability to easily move accounts and communities. Mastodon has that for users already.