You've got a lot of posts here so you probably already know this, but just incase expedia continues with their inability to change the ticket - the airline can edit the reservation on the day of your flight. So if you call within 24 hours of your departure date/time, the airline can make changes. They may charge a service fee (like $25) above the change fees but at least they understand their own rules really well

I just recently had to do this with a BA itinerary because expedia was just too confused about how to do it. It was a more complicated situation because I had already flown one leg but they couldn't understand that the ticket could still be changed without a fare difference if the booking class stayed the same. They kept saying "your ticket has no value left", after speaking to about 5 different customer service agents, I just decided to call BA on the day of the flight and the change was easy once I found a similar fare class via expertflyer and asked them to switch to that date.

EDIT: Oh, sorry I thought you wanted to change your dates. If you got a voucher already from Expedia then that's totally different. There are USA-EU sales every now and then, just keep looking at the Premium Fares Forum