We would never tell you to go the Credit Card forum for a Marriott CC question.
But we might let you know that quite a bit has been discussed about upgrading a current Signature Visa to the Premier card. Check out this thread (the upgrade info begins around post #31):
http://www.flyertalk.com/forum/showt...3&page=3&pp=15
Of course this is a fairly long thread, but there is a lot of information in there. It just may take a little bit to sift through it all first though.