When it's not free dining, people have reported occasional success doing a straight trade at Downtown Disney restaurants and some park places. The most success on this exchange has been reported at your resort counter service but even there it's not a guaranteed trade. I have been in line behind people who were being told they could not use the table service credit because "my computer doesn't recognize the table service credit, sorry...it's saying you have no quick service credits left." The people were referred to talk to the manager and stepped out of the line so don't know if the manager was able to over-ride it for them or not. Just be prepared for the possibility of a hassle.
Unless you have the Deluxe plan...in which case, the credits are meal credits and can be used at table or counter service, either way.