i did exactly this the other day. called up the 1k desk and up'ed to a W fare, immediately applied GPU and the change fee was waived, with a thank you for being a 1k. if you find the agent won't waive the fee, then hang up and call again. beware that in addition to the fare increase, there might be some country-specific premium fare charges (i'm doing cdg-ewr and had to $49). enjoy!