Assuming the migrating sites are successful enough to migrate to SE 2.0, what Area51 phase should they start in?
Having founded a Stack Exchange 1.0 site, Basically Money, that was recently migrated into SE 2.0 as money.stackexchange.com, here's my $0.02 on the subject:
Stack Exchange 1.0 sites migrating into Stack Exchange 2.0 should still have a private beta, but the private beta MUST include all of the SE 1.0 site's registered users. .. (in addition, of course, to the new users that committed to the area51 proposal.)
More in my answer here.
In a nutshell: There's value in a private beta, but don't slam the door on the SE 1.0 user base.
Well, they are public already, aren't they? I would say that, if they meet the necessary criteria to survive like number of users and size of traffic, this should be a rather seamless thing for the users. Therefore I would say they start as public site right away, just with a new, slightly different feature set.
All sites that do not meet the necessary criteria can easily undergo the full Area51 proposal process and save themselves by this time generating more interest. In this case, the mods of the old site should have the possibility to clean up the question body which then should be migrated to seed the beta phase.
At this point I don't have a definite opinion on what phase the sites should start on, but I thought it might be useful to post some information on how it is working at the moment. There has already been one migration and there a few proposals that could be migrations.
The site that made the move was called Basically Money (now the Personal Finance and Money beta) and it went through the same stages as the other proposed sites. There is also a current Libraries proposal in the commitment phase that is a proposed migration from Unshelved Answers).
Of the proposals in the definition phase I am aware of at least two that appear to be based on existing 1.0 sites:
There may be some others, but these are the only ones that I have seen so far.