As far as I know, a ticket booked via a travel agent can only be upgraded by paying the full difference between WT+ and CW. There is no upgrade offer in MMB. This will have to be done via the TA.

Within 24 hours of departure you may be able to upgrade on MMB or at the airport if there is availability.

You can trawl through this thread to see if you can find upgrade offers for IAD:

http://www.flyertalk.com/forum/briti...ng-thread.html

And don't knock the 767 - you either have an aisle seat or a window in CW (and WT+) and guaranteed AVOD. ^