Same Server, Changing Domains, Profile Question

We are going to be changing our existing Win2003 Terminal server from domain "X" to domain "Y". The users currently on the system will be transferring to a new building (and new domain) with the terminal server. How do I copy their profiles such that they can still log into the terminal server with their same username and password but under the different domain "Y".

In advance, thank you for your help.