We've used the
DDP multiple times since they started it in 2005. Every year during the
free dining period (except the first year) Disney has allowed some sort of substitution of CS for TS, due to restaurants being so full during that time and ADRs being impossible to get. It is usually something like trading a TS for a CW plus a lanyard or like this year a CS plus two snack vouchers. They have never allowed two CS for one TS swap though.
During the times when
free dining is not being offered there have been reports of some
resort food court managers overriding the system and allowing a one CS for one TS. There have also been reports of managers refusing to do this though. I have seen both. At POP a few years ago the family in front of me was trying to get some CS meals to take home on their last day and kept arguing with the CM that they knew they were allowed to use their left over TS credits for CS. A manager had to be called and he basically said no, that they were just going to lose those credits. The family was not happy. I was at AS Sports a few months later and the family in front of me was in the same situation (last day, trying to use TS credits before they left for home). Again the CM told them it wasn't allowed, a manager was called, but this time the manager agreed to override the sytem, making a "one time exception". This manager explained that the DDP was set up originally with Disney knowing and expecting that some people would have unused meal credits and that was factored in, when they decided on the pricing.
Except for this year during free dining, I have never heard of a
park CS restaurant allowing a TS credit to be used for CS, (only resort food courts). This year though it was announced just before free dining started that you could exchange one TS credit for one CS meal, plus either two snack credits or a mesh bag. We did it a couple of times when we were there in Sept.