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.
Easiest and what most employers do is allow you to provide two CC's to your corporate TA, in this case AMEX. AMEX issues a C ticket and charges what would have been Y to your corporate account which you apparently submit for reimbursement. It then charges the balance to your personal CC.
All depends on your employer's policies, whether your employer has a corporate discount and what your employer's contract with Amex Corp. allows Amex to do. UA won't change the outbound segments and will direct you to Amex. Amex, of course, has the capacity to make the change, but whether it's allowed to under its contract with your employer is something you need to check. Hopefully suggestion above works. If not, call your headhunter -- time for new employer.