I just did this, so yes you can. You contact WDW ticketing at the phone number on the bottom of the letter your tickets came on. When I called the regular reservations number they did say no you could not change so make sure you call WDW Ticketing instead.
I had to send the Christmas tickets back to them and write a letter telling them what I wanted instead and why I needed to change. I was also told to send my credit card number without exp date so they could give me a credit for the difference in price. I never expected to get a credit for the difference.
I sent the letter by registared mail and now am waiting for the Halloween party tickets. I actually called a couple of times to see if I could do this and again to get the physical address and both people I spoke with said it was no problem. Good luck.