A successful fork needs more than just the content, software and sufficient
hardware, it also needs a community.
If we are serious about having a right to fork we need to make it easy for
editors to keep their account, and possibly even userrights in both forks,
otherwise whichever fork you have to create a new account for is at a huge
disadvantage. But for privacy/security reasons I don't think that WMF should
give the fork a copy of the databases that includes the userids and their
logins. Perhaps this could be finessed by having the WMF create a bridge to
allow wikimedians to activate their existing account at the forked wiki, and
the forked wiki would presumably not allow editors to otherwise create
accounts using names that had edits imported from Wikimedia.
BTW I'm not advocating a fork at this juncture. The only scenario I can see
in the short term that might lead to a fork is the clash between the
Foundation's policy on openness and the contrary decisions taken by certain
parts of the community, - for example EN wiki deciding to restrict new
article creation to Autoconfirmed users. Presumably the Foundation will get
the devs to code the change requested by EN wiki even if it does make us
less open. But it could quite legitimately say "That clashes with our core
values so we won't do that here, but if some of you want to create a more
deletionist wiki you do of course have the right to fork."
In that scenario I'd want the option of keeping my username on both forks,
though I doubt if I'd be active on the spinoff less open pedia. But I'd be
annoyed if they let someone else activate my account there.