I think it's like others have said, if you have it in writing (or, next best, can point them to exact time/date of phone conversation), then you should be able to get that honored. It may take some persistence, but you should get the desired outcome.
I had a rep help me in store with some overage issues once and he said he would back-date my contract change to the beginning of the month so I wouldn't get pro-rated minutes and be clobbered by per-minute fees. He provided me with a receipt that spelled this out. This was not honored when the bill came, so I went back to the store, contract in hand, and played a waiting game with another rep for maybe an hour until they came around. I kept pointing out that they had signed a contract with me, that they expected me to honor my end of it, and that I expected the same of them. I nearly lost it towards the end but did finally get what they had committed to.
Good luck to you.