Can i change phones from a motorola razr to a gravity (tmobile) ?

i have a 2 year contact with them . it ends december 2009 . But i want to change phones N0W . can i buy the phone %26amp; keep the plan and phone number ?. by the way i am on a family plan with my family. (4lines). i have a razr :( and i want a gravity for a couple of months well until my contact ends. so can i ? if so how much will it cost ?Can i change phones from a motorola razr to a gravity (tmobile) ?
Yes, you can change phones, simply swap the sim cards out, the phone will cost you money though, just shop around, they're cheaper online. I'd look at ebay