Sounds weird to say I stumbled across this episode just now. But I did. The coding in the title indicates that it was Season 2, Episode 11: Guess Who’s Coming To Christmas.
I see what appears to be a Chuck older brother. Surely not the one in the Pilot episode. Do we have here another example of Partridge Family Son Syndrome??
I loved this show, and yet wasn’t aware of the Chuck-squaredness issue.
imdb credits Gavan O’Herlihy (he played Chuck) in nine Happy Days episodes. Gavin continued acting after leaving Happy Days.
Maybe the leprechauns took Chuck away? 
Indeed, that’s who I thought was Chuck. So this other guy was a poseur !!!
The official TVTropes-sanctioned name for it is Chuck Cunningham Syndrome. (And the page does not even list The Partridge Family as an example.)
Odd. It’s a prime example. IMDB shows both Jeremy Gelbwaks and Brian Forster ( the great-great-great grandson of Charles Dickens, interestingly) as playing Christopher.
The Other Darrin is a different trope.
Sounds identical to me.
I think we have an overlap: the alt-Chuck you link to in the OP appears to be an example of The Other Darrin–another actor switched into the role without comment or explanation. According to the IMDB cast listing, at some point, they switched between Gavan O’Herlihy and Randolph Roberts. It’s a little tenuous, since Roberts only got 2 episodes, but it fits the trope.
That’s separate from Chuck’s position as trope namer for Chuck Cunningham Syndrome, in which a character simply disappears without explanation or comment. (As opposed to being Put on a Bus, which is when a token explanation given for the character’s absence.) The character was just retroactively erased from the show. Since he’s a trope namer, people tend to jump to the conclusion that you’re referring to “his” trope when you talk about him in trope terms.
nobody made this fuss over Lassie.
My theory is that Chuck disappeared because he was interrupted while trying to steal the wheels from Marty McFly’s DeLorean. Chuck hid in the trunk & was taken back in time to the Old West. After a series of adventures, he was hanged by Augustus McCrae & Woodrow Call.
Damned sodbusters.
Imagine being the kid whose Daddy got a new job…and was told he’d no longer be on television. I feel for Mr. Gelbwaks, who now works in the IT field.
Brutal.
