The nitty-gritty is that NO, you can't have the code applied to your existing reservation number, but YES, if a code-priced room/package is available at the same resort for the same type of room for the same days, then you can book that and then cancel the original one. You won't have to put down a new deposit (they will transfer from the old reservation), and if you've already paid in-full, they'll actualy send you a check for the difference!
There's no guarantee that you can apply a code.
For example, a great code came out for last fall. I'd already booked POFQ -- paid the $200 package deposit, but hadn't yet paid the balance off. I called as soon as I found out about the code. I was tld there were no "code" rooms available at POFQ during my dates, but there were "code" rooms available at POR.
We had our hearts set on POFQ, but my wife and I asked ourselves, "Would you pay an extra $378 to stay at POFQ instead of POR." We reluctantly said we wouldn't pay extra, and opted to switch to POR. We saved $378.
HOWEVER, I called back every single day, and about a week later I got really lucky, and a "code" room became available at POFQ, so we switched back!
So we eventually got the resort we originally wanted AND saved the $378. I love happy endings.