Windows 2000 > Windows 2003 Migration

I currently have a W2K AD native mode forest with 4 W2K AD domains. These domains are not child domains, they are independent domains within the same forest. I have 4 Exchange 2003 servers that are in each domain. I have 1 Exchange 2003 server that is configured as a Front End server. This front end server is in the root of the forest. I have 1 Global Address List that is buitlt from all of the domains. I would like to migrate to Windows 2003 AD but I can’t do all domains at once. Does anyone see any issues with upgrading the 1st domain that has the front end server to 2003 AD and leaving the other domains on 2000 AD? These domains would be upgraded later. Possibly at least a year later. Thanks in advance.