I've never tried to convert CS to SC but sounds good to me! I have, however, treated other guests to a few meals.

We always seem to have CS and SC to use on our last few days. We share alot of CS meals and eat breakfast in our room mostly. Last year I figured we would leave with 2 CS on our cards. As we were checking out at Gasparilla Grill (at GF) where we picked up some food for the plane (also a good idea)

the family in front of us was going to be 2 CS short. I just happened to have that many left so I offered to buy those meals.
A couple of years ago I approached a few families at AK and offered to buy their meals. I bought a family of 4 their lunch at Flametree and the mother gave my a big hug with tears in her eyes and said "Thank you, so much" It made me feel so good, even now I feel good about it years later.
p.s -don't tell anyone but I did leave an unused TS last year. My DNiece had been sick

all night the night before we left and she had to skip breakfast. It never occured to me to try to convert to something else, but that's because we were worried that she wouldn't be able to fly home that day. Now I know to convert unused TS or CS, if I can. And I've learned the hard way to not wait 'til the last day to use up our SC. Last year it took over 30 min. for the CM to get the cash register to work right at Goofy's at DTD. I started getting anxious that we would miss our plane!