I just did this last month. I had old never used one day WDW single park tickets, probably from 1994. I wanted to see the CA color/water show, and spend the day in DL. I presented the two tickets, and asked to upgrade them from one day one park tickets to one day hoppers. I also gave the cast member the option of taking 1 day off of some WDW non-expiration hoppers I had.
I was willing to use either ticket, I told him to do whichever was easier for him. He opted not to "mess with" the non-expiring tickets, and used the 1 day 1 park tickets. I fully expected to be given the value of a
Disneyland 1 park ticket and pay the difference to hop; but he just gave us 1 day hoppers and told us he was a nice guy.
Originally I waited in line at the ticket window-where you purchase tickets. That was the wrong place. The main entrance to DL or CA is the place you need to go. Once you are in front of a person you ask for a "lead" or "supervisor" and explain to the turnstile person you want to use WDW ticket media. They will ask you to step aside, and call someone who can get the whole thing sorted out. He took the tickets and went somewhere to confirm the available entitlements. It probably took all of 10 minutes. I was just glad to use up a couple of the old 1 day tickets I had (purchased with vacation packages to get other perks over the years).
I sort of expected the whole thing to be a giant pain, and was prepared to just buy entrance tickets. I was pleasantly surprised at how easy the process worked out. In case you collect these things, they did return my WDW tickets from which the value had been removed, but they had little holes in them to prevent future scanning or bar code reading. Personally, I just threw them away, but I know some people scrap book, etc.