Ok, here is what I did. I also had a sidekick ii and wanted to upgrade to the 8700. Believe it or not, you can do this, but only if you cancel your current contract and through some hassling.
I got my 8700 through Amazon for free with the t-mobile and amazon rebates. I also got a RAZR for my wife for -$50...At any rate, I had two accounts then. One for the sidekick and one for the blackberry. After MANY phone calls, I finally found someone who was smart enough at Tmobile customer service to figure out what I wanted to do.
Tmobile cancelled my sidekick account (I was not under contract) and actually ported the phone number to the blackberry. This was the difficult part and it takes some time and patience if you want to get it accomplished. But make no mistake about it, you can definitely swap your phone numbers on accounts. They initially tried to tell me that the number (once cancelled) goes into a pool and you are not guaranteed it, but it's BS. The CS rep did it in 5 minutes for me. AND, I received both my tmobile rebates and Amazon rebates because the new account is still active. You just lose all of the good will you might have built up on the old account. If you were a customer on that account for 10 years, it basically goes away and they treat you as a new customer.