what you're wanting to do is not possible w/o some sort of user interaction (separate SRPs). here's some threads - i'm sure there are a lot more available on here and on blackberry.com's knowledge base.
http://www.blackberryforums.com/showthread.php?t=10406
http://www.blackberryforums.com/showthread.php?t=8536
FYI, I've gone through migrations using multiple SRPs (when I didn't know any better) during our Exchange 5.5 to Exchange 2000 migration a few years back; as well as during our beta/pilot to production hardware migration (for our parent company's BES). If you purchased a second SRP, you might as well use it as a test environment (BES 4.1 to be released next week) and use your current SRP on the new BES (and truly migrate without user interaction or disturbance).