I know Dennis James once filled in for Bob Barker back in 1974 at Christmas as Bob was ill and unable to make the taping.
Geoff Edwards filled in for Monty Hall on The All New Let's Make A Deal in the mid 1980s as well as pinch hitting for Bill Cullen in 1980 on NBC Chain Reaction.
Arlene Francis of What's My Line? fame was a substitute host for Bill Cullen on the 1950's Price is Right.
Jim Peck for Jack Barry on The Joker's Wild syndicated version.
Bill Cullen filled in for Alan Ludden on Password Plus in 1980.
Vanna took over the reigns of Wheel when Pat was going through his health challenges.
I know Scrabble had a game show host week where the different hosts took turns hosting the game so that Chuck could play. Ditto Bert Convy and Tattletales.
Dennis James also filled in for Monty Hall on the old Let's Make A Deal in the late 60s or early 70s as evidenced by the New Price is Right pitch tape featuring Dennis and Mark Goodson.
During Game Show week in I think 2005 Peter Marshall took over during the main game in one episode from Tom Bergeron.
Alex Trebek pinch hit for Chuck Woolery on Wheel for one week when it was on NBC.
Sajak and Trebek switched roles for one episode of Wheel and Jeopardy on 4/1/97, but that was more a fun thing than a necessity.
I'm trying to think of others, I'm sure I'm missing some. Oh yes, Bill Cullen also filled in for Garry Moore on To Tell The Truth during its 1970s syndicated run, I know of one episode where Bert Convy pinch hit for Bud Collyer, and also Mark Goodson himself was a sub host for Alex Trebek for the 1990s TTTT on NBC as Alex's wife was having a baby at the time of taping.
