oabgen differential errors while installing exchange 2010/2007 into a 2003 organization

If you are sure everything is ok, public folders are replicated, servers have been changed in the receipient policies, lists have been upgraded, etc etc, you must create

'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeSA\Parameters\OAL post full if diff fails'

DWORD, value 1

and update/rebuild the oab. It should be ok afterwards (verify by setting logging level to medium.
Remember to remove key afterwards.

How to set logging level?

Set-EventLogLevel -identity "MSExchangeSA\OAL Generator" -level medium