New 2012 domain in an existing 2003 forest

Hi,

I currently have a forest with many tree domains, I have a mixture of 2003 & 2008 R2 domains and have recently added a new 2012 DC to the forest, the current domain function level for the forest and all domains is Windows 2003 Server.

I'm adding my first 2012 tree domain to the existing 2003 forest but I cannot reduce the domain function level below Windows Server 2012 for this new tree domain, when running the Active Directory Domain Services Configuration Wizard.

My 2008 R2 tree domains are all running at the Windows 2003 Server domain function level.

Is this by design?  Am I going to run into any problems with the new 2012 tree domain DC replicating with the forest 2003 DC's?

Thanks