GSN aired some early syndicated TTTT episodes where if a member of the panel would've "disqualified" him/herself from questioning, they would switch places with Garry Moore, having him question the team of challengers. In one case, Bill Cullen was subbing for vacationing Garry Moore, and Peggy Cass traded places with Bill for the one round.
Among other occurrences, I recall Chuck Woolery playing Scrabble, during a "Game Show Hosts," week for charities, while one of the other participants occupied his podium. The only one I recall seeing do so, was Marc Summers.
The EOTVGS mentions Bill Cullen playing The Price is Right for one week, while Robert Q. Lewis hosted.