Do you have another day in mind where you would like to use them? I would call Disney customer service. They have that flexible one day ticket option that costs extra and lets you use the ticket anytime that year. They I am guessing would let you change it to that at least.
I haven't personally had that experience, but as Robo above alluded to, I had a situation where I had bought one day tickets for March that I changed to be applied to a three day ticket in January due to changes in travel plans. It's not like I could make those change online, but calling Disney they made those changes for me.