when you try to prepare Sechma for Exchange 2010 RTM in a exsit Exchange 2007 SP3 environment, such error will show:
Setup encountered a problem while validating the state of Active Directory. The Active Directory schema version (14625) is higher than the Setup’s version (14622). Therefor, PrepareSchema can’t be executed.
this is a known issue that the schema version in Exchange 2007 SP3 is higher than Exchange 2010 RTM.
|Exchange||Forest (rangeUpper)||Forest (objectVersion)||Domain (objectVersion)|
|2010 SP1 (beta)||14718||12640||13040|
So the best way is:
If you have update your Exchange 2007 to SP3, what you have to do is waiting for Exchange 2010 SP1. it is said that it will released at 2010-09-01.