You will need to finish paying off your old phone depending on how much the new phone you want is. T-Mobile allows you to put up to $720 worth of phones on EIP per line.
Above is part of what I said. You have a limit as to how much you can put on EIP. It sounds like you wouldn't be crossing that amount, By the way if, you don't have enoughEIP available they don't make you pay towards the old phone, rather they increase the up front cost to make up the difference.