You should consider using the Small Business Server Transition Pack to move out of the SBS model and into the “full” product model. Once you do that, you can install a second Exchange Server 2003 on your network and move mailboxes over to it. And having done that, you can then follow standard practice to remove the original Exchange Server from the orgnization.
The transition pack is required because installing a second Exchange Server in a SBS network will cause problems for the wizards and key operations of SBS management.
You will find the following information helpful:
1. SBS Transition Pack:
2. Removing the FIrst Exchange Server from an organization: