Hunnybear - I just ran into that exact problem with United tonight on a ticket purchased about three weeks ago. Problem is that altough the original ticket was in W and the new discounted fares are in W, the agents (I tried twice) wouldn't issue a credit unless there is a free seat on the flights in W so they can issue a replacement ticket for administrative purposes. (They wouldn't actually issue a new ticket and there wouldn't be a change fee but they still had to have a seat available). Since W was sold out on my flights they wouldn't issue a credit.

So much for United customer service.