it really depends on why you want to upgrade and from what kind of phone. if you are coming from a non-smartphone, then they'll probably bend over backwards for you. if you already have one, then they'll force the full price on you.
here's a tip I used when I was in a similar situation - ask them how much it costs to terminate your contract. it will probably be $375 pro-rated, so about $300 after 5 months. then, if the cost of the new phone you want is free or $300 less than the full upgrade price, terminate your contract, then sign a new one. tell them that is your plan, then they might let you slide.
good luck.