I did it in 2003. I first got in line at the tickets booths and then they sent me to the turnstiles to have to ticket converted. A CM took the ticket and came back a few minutes later with a DL ticket.
However, in those days all tickets came with no expiration and park hopper privileges. If your current tickets have expired you probably wouldn't be able to use them at all. If they don't include park hopper then you would have to choose between DL and DCA.