Change web host for an application with constant user data updates

How do you change web hosts if you have an application in which the database, say, discussion forum, is being constantly updated by the users?  It seems like when the DNS is propagating  a user might update data on either the old or new version, and then either be out of sync with themselves or with other people in the conversation. I appreciate you may have to have a downtime for cut over, but that doesn't seem reasonable to have that be two days waiting for DNS propagation. I know this must be done often in the real world so what's the trick?