Originally Posted by SEA1K4EVR

Unfortunately you can only do this if your Y ticket is changeable/refundable without a change fee. Years ago UA would let you just pay the fare difference but they changed most of the fares so that you'd also have to pay the change fee.

But, if you're on a flexible fare you should be able to do it. Since you booked through AMEX if you call UA to change it most likely they will tell you that you have to go back to AMEX to make the change... which is probably fine.. the AMEX travel agent ought to be able to accept your personal credit card for the difference.