Question : Raise Active Directory domain functional level to Windows Server 2003

We are preparing to install a new server with Exchange Server 2010.  I must upgrade my domain functional level to Server 2003 or higher.  Currently all of our primary DOMAIN CONTROLLERS are running Server 2003, however we have several member servers of this domain that are running Server 2000.

In addition, we have a second domain, and older one that is being used for histrorical access to certain application servers on the old domain that is an NT server.  Members are not logging into the computers to the old domain, but rather have user accouns setup for application access to a server on the old domain, which is why we have to keep it up.

Also, the current domain functional level is Windows 2000 native and the forest functional level is Windows 2000.

Will raising the domain functional level to 2003 affect anything with the OLD domain?  There are currently both incoming and outoing trusts on the new domain that are going to the old domain.

Answer : Raise Active Directory domain functional level to Windows Server 2003

Raising the functional level of the forest/domain only affects down-level DCs(!) for the forest/domain; domain members and trusts aren't affected by this.
In other words: yes, you can have a W2k3 AD running in full functional W2k3 level, with NT4 domain members (just not NT4 or W2k DCs) and trusts to NT4 domains.