Hi -
I actually did this today. I had originally booked my USAir flights on internet only fare, and today they dropped each $20 for a total savings of $80, so they are sending me a voucher for the differencce. This new fare was also an internet only fare, so I don't know if it applied to me since I already had an internet only fare, or if anyone could have called and received that reduced fare. The one thing I do know is that the fare has to be available on your EXACT itinerary, so make sure it's available on the flights you have already booked.