Hosts returning to their show as a guest

[aaron sica]

Then you have the reverse version, where someone who will host the show appears. (see: Bob Eubanks promoting All-Star Secrets on Card Sharks)

You could also have two hosts of a show, past and future, appearing on another show (see Jim Perry and Bob Eubanks, on the same team on a celebrity week of "Family Feud").

[ChrisLambert!]

I would watch a documentary about Allen Ludden's surprise walk-on on Liar's Club, then ending up replacing Bill Armstrong as host on it soon thereafter.

[knagl]

Jack Narz announcing Monty's  "Beat the Clock"

If we're counting that, then Marc Summers as announcer/sidekick on the 2018 Liza Koshy revival of Double Dare.

[DoorNumberFour]

Then you have the reverse version, where someone who will host the show appears. (see: Bob Eubanks promoting All-Star Secrets on Card Sharks)

On this subject: Trebek showed up as a (non-playing) guest twice on Jeopardy: March 7, 1974 and at some point in the first week of April 1974, to promote The Wizard of Odds.

[JasonA1]

The reverse is interesting. Joe Farago performed in the Prize Vault several times before taking over as host.

-Jason

[BrandonFG]

I like the reverse idea too. John Davidson appeared on Marshall's Squares a few times, and he even contributed an infamous blooper that's made its way onto a couple specials.

[Jimmy Owen]

I would watch a documentary about Allen Ludden's surprise walk-on on Liar's Club, then ending up replacing Bill Armstrong as host on it soon thereafter.

  Considering that Allen and Bill had worked together before, that would be a "Stumper"

Bill later announced the Eric Bordman version , though John Barbour introduced Eric.

[DoorNumberFour]

, and he even contributed an infamous blooper that's made its way onto a couple specials.

On the whole, that’s a great clip.

[JMFabiano]

Then you have the reverse version, where someone who will host the show appears. (see: Bob Eubanks promoting All-Star Secrets on Card Sharks)

On this subject: Trebek showed up as a (non-playing) guest twice on Jeopardy: March 7, 1974 and at some point in the first week of April 1974, to promote The Wizard of Odds.

Did someone mention Tom Kennedy as a player on Password Plus?  Regis Philbin, later on? 

[doctorwho]

Re: the reverse happening, Meredith Viera on Celebrity Millionaire with Regis comes to mind.

[TimK2003]

And from the "What Almost Was (In Reverse)" category:  Bert Convy hosting the pilot for Match Game '90.

[Casey]

Then you have the reverse version, where someone who will host the show appears. (see: Bob Eubanks promoting All-Star Secrets on Card Sharks)

On this subject: Trebek showed up as a (non-playing) guest twice on Jeopardy: March 7, 1974 and at some point in the first week of April 1974, to promote The Wizard of Odds.

Did someone mention Tom Kennedy as a player on Password Plus?  Regis Philbin, later on?

Bert Convy also played Password Plus.

[MSTieScott]

Re: the reverse happening, Meredith Viera on Celebrity Millionaire with Regis comes to mind.

And Jimmy Kimmel.