It could be a cast addition, an abandonment of its initial premise, or an altering in a character's personality or MO.

See also:
https://www.urbandictionary.com/define.php?term=Riker%27s%20Beard

http://tvtropes.org/pmwiki/pmwiki.php/GrowingTheBeard/LiveActionTV

Replacing Steven Kampmann and Jennifer what's-her-name with Peter Scolari and Julia Duffy on Newhart.

Adding Dawn Wells and Russell Johnson into Opening Credits by the beginning of Season 2 of Gilligan's Island

The Odd Couple filming in front of an audience sharpened the performances of Jack Klugman and Tony Randall, and made the show much better.

In Green Acres it started out with Lisa being level-headed and Oliver was the silly one. Then Oliver became the straight man and Lisa turned odd like the rest of the town. This was a pretty substantial change but a great improvement, IMO.