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?
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codequestAsked:
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Jason C. LevineDon't talk to me.Commented:
Two days is an extreme.  DNS propogation usually takes effect in as little as 4 hours and is complete in 24 hours.  So a downtime for cut-over is not as bad as it seems.  Take the old site down, leave a "we're moving" single page up and get the new host spinning.

The more difficult path involves merging the two databases.  Not recommended unless a downtime would cause unavoidable harm/loss.

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codequestAuthor Commented:
Thanks for the input.  Hmmm.   The app I'm working on could (theoretically) have some heavy business use that would not be happy by 24 down time.  But I guess by the time we got there we should have already migrated to a major host and settled into it pretty firmly, so that the chances of changing host at that stage would be small.  And if that were to happen, people a lot smarter than myself could deal with it.  

(this link seems to fall under the header of "people a lot smarter than me" :-)
https://www.braintreepayments.com/blog/switching-datacenters/
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