2003 domain & 2012 domain in same subnet?
Hi all. I have a friend who has an old SBS 2003 server that runs Exchange 2003. He's looking at replacing that system with a new server running Server 2012 Standard and Exchange 2013. I know there's no migration path from 2003 to 2013, but what I'm wondering is if there'd be an issue having Exchange 2013 up and running and functional in the current environment with SBS 2003?
His domain is also a .local internally and because of the issue beginning in 2015 with SSL certs and internal domain names I'm going to use this new server as an opportunity to completely replace his existing internal domain, Active Directory, etc. So adding this new 2012 server to the existing domain, making it a DC w/DNS, etc. to more easily migrate over to using the new server isn't an option.
Knowing all of this I'm wondering something. What would be the implications if I connected the new server to network, gave it a static IP, created a new domain in the same IP subnet as the existing domain, installed Exchange 2013, etc.? I believe I should do this on a separate subnet but am just wondering if it would really cause any issues? All the PC's are members of domain.local and I'd be creating domain.com.
What do you all think? What issues do you see that would/could occur in this scenario?