Hi, I am with TM and upgraded to the BB8700g in June. I had just bought a phone in April so the only upgrade I was eligble for was $350 with a 2 year contract (which I didn't mind as I like TM service). A steep amount but I really love my BB.
As existing customers we don't get the deals that new customers get but after 11 months of service, we do get those upgrade deals. So you may be eligble for something. No offense to Cingular, but I would stick with TM because they offer the best plan...$19.99 BBAddon (when you add it to your regular phone plan) for unlimited data OR $29.99 without the phone plan.
And no, I'm pretty sure that even if you cancel your current service and buy a new one that they will then consider you a new customer. So I wouldn't recommend that. My final thought would be to call TM customer support and see if they can do anything for you. Your contract is up so they might make some special offer to keep you as a customer for another few years. Worth a try...