Multiple SBS servers on same domain

We currently have sbs2000 installed running Exchange & SQL and our ERP system. We are in the process of replacing our ERP system with MS CRM. We will be installing CRM on a new sbs2003 server running SQL and Exchange. The SQL will be all new, but the Exchange will need to take over for the old exchange. The problem is this: I can’t just replace the old system with the new. The systems will need to run side-by-side. Users will be accessing both systems during transition. This should only be 2 weeks or less, but they will need to run on the same domain.

Any ideas, suggestions, or otherwise?