Not sure when you're going, but be if you try and exchange them at the F & W booths, some will do it, and some won't. We were told at POFQ that we could exchange a CS for 3 items at the same booth.....but only 3 out of the 7 we tried, would do it. But we were able to change them at all the other places we tried.
They can all do it. If they try to tell you they can't, they just don't know how. (Mention the
DDP Combo button if need be) We just got back, and we had two things going for us: A pair of premium packages for M-Th service in the premium areas, and the
Disney Dining Plan. We used the VAST majority, if not all of our QS credits and most of our snack credits in the F&W areas, and while many cast members weren't as familiar with how to process that conversion, it's totally possible. We used two QS credits at the Chew Lab, and another in Brazil (2 pork bellys and an order of that cheese bread make a nice QS credit use). We also helped educate several of the premium package area CMs on how to do it, and managed to get several QS credits to be worth about $18 in food in the process.
The keys to making this most successful are:
1. (and this one is essential) You must be getting all three items in the same transaction (in regular F&W, this is a per kiosk limitation; in Premium areas, all kiosks you want items from must be served by THAT premium area)
2. Keep the types of payment separate: 3 snacks for 1 QS = 1 transaction, then do the two items you wanted to use for snack credits, then do the things you wanted to use that gift card for, then order that drink with the sampler or premium area tab. You'll have a million receipts, but it's okay.
3. Put value on a gift card for OOP expenditures, so that it's one less opportunity to goof with the magic band transaction. We went ahead and got the $200 commemorative GC and used it all over the place for things we would have otherwise used our room charge to pay for. It worked well.
Be persistent and they'll get it for you.

Good luck!! Enjoy!!!