We're adding an additional Exchange server at another physical site whose subnet (2.2.2.x) currently exists in a different AD site (SiteB) than our mailbox server site (SiteA). We cannot eliminate the AD SiteB, so we were thinking of splitting the AD subnet into different smaller AD subnets. We'd likely split them into .0-239 and .240-245 or so, then move the smaller subnet into SiteA as most of the in use addresses would fall into the former range. I noticed that you can create overlapping subnets so the plan would be to create the new subnets and leave them in the current site, move the smaller 240-245 subnet to SiteA, then delete the /24 subnet. Is this sound and what effect if any would this have on existing machines? Thanks.
AD will match the most specific subnet object. "240-245" isn't something you can chunk up into any sort of valid subnet object.
What you need to do here is define /32 subnet objects on a per IP basis. My suggestion is that you leave the existing subnet object linked to SiteB, and then create /32 objects for 240-245 in SiteA.
I'm not aware of any problems with what you're looking to do. You want your AD site topology to describe your physical network topology accurately.
If domain controller computers will be moving to a new site you'll need to move their computer objects in the "Active Directory Sites and Services" console. Client computers and member servers will be unaffected.
Exchange 2003 will be unaffected.
Exchange 2007 routes inter-server traffic based on AD site topology. If you're leaving the Exchange Server computer's location alone there will be no effect.