Oddly enough, I actually think this might well be possible, and using native options within the SEPM, no less!
Let me caveat this by saying I've not actually tested this out, but it should (in theory) do the trick...
The SEPM's Auto-Upgrade function provides the option to point clients at an alternative location from which to download client packages:
http://www.symantec.com/docs/TECH97406
All you really need to do is enable this on the old SEPM, and point the the group at the new SEPM using a link created using the "Web Link and Email" function in the Client Deployment Wizard:
http://www.symantec.com/docs/TECH204046
And there you have it, clients managed by old SEPM, pointing to new SEPM for a client installation package.
If you have a chance to test this out, I'd be really curious to hear your experience, as I'm unsure if upgrading in this manner will correctly adhere to the "Remove all previous logs and policies, and reset the client-server communications settings" option you must set (if you want them to redirect) when creating the Web Link through the CDW on the new SEPM.