You don't actually "convert" the credits. You can use a TS credit to pay for a QS meal at just about any QS place, but all your QS credits must be used before they will do that.
There are some places where you can gather 3 snack items and use a QS credit to pay for those 3 items. You can't do it everywhere, all three have to be snack credit eligible, it must be done in one transaction, and personally, I would ask before I gather my items to be sure it can be done.
When we did it in 2015 it was mainly at the resort. The CMs at the food court and gift shop at POR were awesome and it was really easy. I'm glad it can still be done