Your only choice is to officially change the ticket, either on the same day or earlier--it doesn't matter much. This means probably a change fee of $250-450 and any fare difference, although some agents may waive at least some of these charges, especially if you have status and especially on the return portion of a RT.

It would help to know your fare class as well as the fare class availability (for example, from EF) for your current flight and the flight you want.