The over-changer was a piece of technology aboard the USS Enterprise in 2265 and 2266. Found with other technologies on a checklist, the over-changer's status was reported to the bridge. (TOS: "Dagger of the Mind")
Over-changer is one of the items mentioned in the background chatter starting with TOS: "Where No Man Has Gone Before", and heard most clearly in "Dagger of the Mind". Other than its existence and its importance to the bridge crew, there is not much more known about this device.