What I mean with "universal login" is one account for multiple Fediverse services, I guess that wasn't clear from my post. Yea, proper migration is hard and questionable if we should even allow it (could cause all kinds of issues, espwcially regarding account security) but Mastodon allows you to move your followers and add a redirect which is the most important part of the account and Lemmy should probably try to do something similar with ranks and communities.