Originally Posted by
MSPeconomist
You could also phone UA to do the transaction and apply the credit from the more expensive ticket toward the sale price ticket, assuming that you see a sale and not a mistake fare.
Yes, this. And definitely call it in instead of doing the change online.
The "correct" way to do the above transaction is to charge you the change fee and then provide you a electronic travel voucher for the fare difference, which is what happens online. But often, when you call in, the agent will simply subtract the change fee from the fare difference and give you a voucher for the remainder, without you having to pay the change fee first.