Specially comments about the migration to PSE. It'a a little weird to see "migrate this question to PSE" here on PSE.
Or it's better to keep the history untouched?
I was thinking that it might be bad to have them deleted because it wouldn't leave a history and people might be tempted to re-migrate the question, but they'll see the "Migrated from StackOverflow" so that's not a problem.
What needs to be preserved is the time of the comments for complete transparency - we should strive for honesty and not social engineering. And in order to preserve that, we'd better just leave the comments alone.
There's a subtle difference between saying "We should move this to Programmers.SE" and "You should have asked this on Programmers.SE" which could influence the tendency for closers to close a given question based on it's own merits.
I think it's better to keep the history untouched, so long as there's that big notice that the question has been migrated.
I think what really needs to happen is that we should scale the votes on the questions based on active users. Right now the top four questions here are all migrated from SO. Many of them weren't even that popular on SO, but they are here due to the user base skew.