Oddly, you can't "downgrade" the
free dining plan offered with the level of resort you are staying at. I think Disney is using this in some way to balance out the crown levels between the TS and CS to some degree during free dining, when the restaurants tend to be incredibly busy.
As for trading TS for CS, that is one of those It Depends answers. You may have more success at a resort than in the parks and you also may need to see, if all else fails, if your guest services desk at your resort will convert the credits from TS to CS. Success with exchanging the credits is varied but it won't hurt to ask.