[ActiveDir] W2K3 Upgrade Domain Controller or Exchange Servers?

[Bahta, Nathaniel V CTR USAF NASIC/SCNA]

All,   We are rounding home base in our upgrade path to 2K3 and have our Exchange Server Cluster running W2K and EXCH2K and our Domain Controllers to upgrade lastly.  Which of them would you think would be the best to upgrade first?  We thought to upgrade the DC's first because it takes care of the extension of the schema and all which has to be done prior to EXCH2K3 anyhow.  I cant think of a reason to not upgrade the Domain Controllers before the Exchange Server.  Can anyone else?     Thanks   Nate