I'd say 3-4 weeks, once folks have had a chance to register.
We're (likely) changing, because the "old" ezBoards run about $300/year, yet we can run this phpBB board software on the web space that we already pay for (~$95/yr) for the website/roster/database, etc.
Also, because the user database will be on "our" server now, we can also do some automated updating between the roster and board access. ezBoard is cumbersome to administer, and that means sometimes folks' access is only removed when we're reminded they shouldn't have access someplace.