I have no idea if this is valid or not, since we never tried it, but according to Mousesavers.com:
"You can use table service meal credits at a quick service restaurant. Likewise you can use quick service credits for snacks (but be prepared for the cashier to think you're nuts). You'll lose some value, but gain flexibility."
The last two years I have been at WDW for free DDP in September and indeed this was an option, primarily due to no TS availabilty for those that did not make ADRs in advance.
Our Authorized Disney Vacation Planners are here to provide personalized, expert advice, answer every question, and uncover the best discounts.
Let Dreams Unlimited Travel take care of all the details, so you can sit back, relax, and enjoy a stress-free vacation.