The last time I did this (Feb 04), I had no problems at all. I did it over the phone (not in person), and just said, "I have tickets for another trip I'd like to cancel and use towards this trip." Don't seem to remember any problems.

In terms of the "was this refundable ticket purchased with a voucher", either you ignore the comment, or else you answer, "I don't remember". Technically the actions of using a voucher to buy a refundable ticket, and then exchanging a refundable ticket to use towards a different ticket are not illegal. One could question the ethics if your sole purpose was to 'launder' or extend the expiring voucher, but there's not definite way they can prove you never had any intention of using the original ticket.

Jeff