Pretty self explanatory. Sitcoms in which a major character was replaced due to the actor playing him/her leaving, dying or getting sacked. The most obvious example is Only Fools and Horses in which Granddad was replaced by Uncle Albert because Lennard Pearce died. And when Buster Merryfield died they (sort of) developed Del Boy's son as a replacement character.
Across the pond, an obvious example is Cheers. When Shelly Long left to pursue a film career after three seasons, they got in Kirstie Allie as a replacement.
And if you know the reasons why the character/actor was replaced, that'd be great.