Used to be that during free dining (when it was only offered once a year) that Disney would announce that dining plan users could trade a TS credit for CS meal plus extras, like an extra snack or merchandise. They don't announce it anymore and they don't have a specific period any longer where doing such a switch is allowed at all CS restaurants all the time. Best you can do is ask at the CS restaurant where you want to use your TS credits. Some of them will accept, and some of them will say you can't. It may actually come down to who you ask.
Most guests who are able to make the switch appear to have good luck at resort CS restaurants. And it's now a one for one trade - for your one TS credit, you will receive one CS meal, no extras.
At many of the resort CS you can trade a CS credit for two or three snacks. This is less likely to be allowed in the parks or at stores and shops. Don't know if you would be allowed to trade a TS credit for snacks.
Just from reading your signature that you are anticipating staying at the Dolphin in 2012...You can't get the dining plan at the Dolphin. So if you're considering a dining plan you will need to change resorts.