ello m8...seems tht u shud be given a good discount on ur line coz like ur bills are over the normal line rental itself...wen u com to upgrade they will give u a discount price, and u can use that to either get a new phone or knock sum money off ur line rental..either way its up to u.
thing is you dont have to agree to it straightaway, just tell them you want to think about it or something and call back like a day or so after and see if they can do anymore on their offer...chances are they might be able to, but being t-mobile and depending on the person ur speaking to, u mite not get anythign more out of them...
good luck tho!