Even if you had to make a physical copy of the DB, run the first pass off that and then point it to the 'live' VB forum for the 2nd pass, that would be fine.
Downtime would then be in the minutes (well, perhaps an hour or so) as opposed to day(s).
I can see the migration of my forum easily taking 8-12 hours on the live server. That's a long time for the forum to be offline.
That would also be a hell of a lot higher if the search indexes had to be rebuilt (which in all likelihood they would be at the end of the first pass).
And my site isn't huge by any stretch of the imagination. (4m posts, 370k threads, 88k members).