Channel Hop: The radio show moved from NBC to CBS in 1949, one of a number of shows and personalities that the latter network "raided" from the former.
The show had jumped around quite a bit in its radio days: starting on NBC Blue (later ABC) in May 1932, it moved to CBS that October, then to NBC Red (now NBC) in March 1933. It went to NBC Blue in October 1934 and back to Red in October 1936, where it stayed until the great talent raid of 1949.
Also, the TV show moved from CBS to NBC for its final season in 1964.
The channel hop also affected Phil Harris' show: For a time, Harris would try to "commute" between NBC and CBS (easy to do as the buildings were very close together) to do Benny's on the latter and his own show on the former. However, NBC got fed up, and demanded he choose — his own show, or Benny's. When he chose his own show, the character of Frankie Remley (a real-life member of Benny's backing orchestra, but played on Harris' show by Eliott Lewis) ended up getting a forced rename, as the character proper belonged to Benny's show — the expedient solution being to turn Lewis into The Danza and have the character go under the name "Eliott Lewis."
The Danza: Most of the main cast, with the exception of Rochester (real name Eddie Anderson) and Mary Livingston (Sadie Marks, though she later had it legally changed to match her character). Mel Blanc is a semi-example as "Professor LeBlanc".
Executive Meddling: Jack's TV show ended because one network executive decided that he was too old for television and told the network to cancel his show immediately.
Irony as She Is Cast: In real life, Jack Benny was actually a very good violinist. It takes a lot of musical talent to be able to play a musical instrument badly for comic effect and having it come out amusing rather than painful.
Mean Character, Nice Actor: Jack Benny WAS this trope. His radio persona was a vainglorious petty miser, who wasn't above taking advantage of his close friends and cast if he could get away with it. Jack Benny, in Real Life, Jack was universally known as a kind and very generous man.
In fact, Dick Cavett paid Benny a posthumous tribute by remarking that after most celebrities died, stories would come out of the woodwork about the deceased's peccadilloes, meanness or other faults. Benny was so universally loved in Hollywood that no one had a nasty thing to say about him after he died.
The Other Darrin: Before Phil Harris, many noted bandleaders of the day provided the music for the show, among them George Olsen, Ted Weems, Frank Black, Don Bestor, Johnny Green and Jimmie Grier. Harris was himself replaced by Bob Crosby (Bing's brother) when the show moved to CBS.
The Pete Best: Several people from the early history of the show. Don Bestor, Johnny Green, Frank Parker. The best example is his writer Harry Conn. Harry Conn was Jack's writer until mid-1936, when he claimed that Jack had no talent of his own and all of his laughs came from his head. Coupled with his wife making a similar remark to Jack's wife Mary Livingstone (i.e., that Mary could only afford her fur stole through Conn's talent), he was fired and left Jack without a script. Jack hired two writers named Bill Morrow and Ed Beloin, who greatly refined the show's humor and the characters into what we recognize until the end of the show. Harry Conn barely wrote anything after leaving the show and wound up as a doorman.
Talking to Himself: One episode featured Jack and Rochester taking a road trip to Palm Springs. A scene in a gas station featured Mel Blanc playing the gas station attendent, as well as providing voice overs for the Maxwell's motor and Polly, Jack's parrot.
Throw It In!: That's what Mel Blanc did when the sound effect recording for Benny's Maxwell failed to play on cue. Thinking fast, Blanc took the mike and improvised the sounds himself. The audience loved it so much that Benny decided to dispense with the recording and keep Blanc doing the sounds himself.
Jack Come to think of it, you look just like Professor Le Blanc, too...
Frank Nelson's many appearances (the roles vary, the character remains constant) also qualify. One episode even has Jack visiting a shrink, convinced that he's losing his mind because he keeps seeing Nelson everywhere he goes!
At the end of that ep, Nelson shows up at the doctor's office, feeling the same thing about Jack and fleeing at the mere sight of him. This makes Jack feel better.
When Dick Van Dyke guest-starred on the show they did a skit involving a murder mystery. Jack was the inspector and Dick was everyone else.