Exchange fellow Johan Veldhuis blogged about something interesting (or rather, something silly) which you should know about when planning to deploy Exchange 2010 RTM in an Exchange 2007 environment.

Apparently, the Service Pack 3 for Exchange 2007 raises the version number of the schema above the version number set by Exchange 2010. This will result in the following error message when trying to upgrade the schema:

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.

The result: You can’t perform the schema upgrade for Exchange 2010 RTM in an environment where Exchange 2007 SP3 is already applied.

Now this information is not only of interest to current Exchange 2007 users, but also to clients wanting to migrate from Notes to Exchange 2010 for instance. They might want to make use of Exchange 2007 for running the Transporter Suite to connect Notes to Exchange. Implementing Exchange 2010 RTM first is also not an option, because that would prevent the installation of Exchange 2007.

Given this information you could assume the following order of installation would prevent this issue:

1. Prepare for Exchange 2007 SP2 sets rangeUpper 14622, forest version 11222 and domain version 11221;
2. Prepare for Exchange 2010 RTM sets rangeUpper 14622, forest version 11222 and domain version 11221;
3. Prepare for Exchange 2007 SP3 sets rangeUpper 14625, forest version 12640 and domain version 12639?

But alas, when trying the perform the PrepareSchema of Exchange 2007 SP3 against an Exchange 2010 RTM prepared organization we are presented with the following message:

The exchange organization does not support this version of exchange server

When I retried the same thing but with installing an Exchange 2007 SP2 server , the operation seems to work, i.e.

1. Prepare for Exchange 2007 SP2 & Install Exchange 2007 SP2 server;
2. Prepare for Exchange 2010 RTM & Install Exchange 2010 RTM Server;
3. Prepare for Exchange 2007 SP3.

I also did an Active Directory compare using ADexplorer comparing the PostEx2007SP2-PostEx2007SP3 situation against the PostEx2007SP2-PostEx2010RTM and it showed nothing of interest really, besides the usual ChangedOn, CreatedOn and GUID differences and some changes which had to do with the order of installation.

According to information in the Exchange Server Active Directory Schema Changes Reference dated June 2010, “Exchange 2010 makes the same changes to the Active Directory schema as Exchange 2007 SP2”. But Exchange 2007 SP3 makes some additional changes to the schema, which can be checked here. Looking at the explanation contained in that article, these changes may be required for environments attaching disclaimers to voice mail or fax messages. Of course, those changes won’t be there when you prepared the schema using Exchange 2010 RTM, so those environments using that functionality might expect issues in that area.

To wrap things up, unless it will become officially supported to run Exchange 2010 RTM in (directly) Exchange 2007 SP3 prepared environments, you have to be very careful with planning the order of installation for greenfield scenarios or scenarios where Exchange 2007 SP2 is in-place. I assume Exchange 2010 SP1 will solve the problem as the Exchange 2010 SP1 beta updates the schema to version 14718 (could change with release of SP1).