Years ago, he said he intended to stay on board until he physically can't hold Big Bird's head up anymore.
Since reaching that point in life, he then began saying he intends to stay on board till Season 50.
As far as I know from speaking with the castmembers at the Fanboy Expo, plans for Season 50 have already been in the works, and knowing how far in advance they tape SS, they'll probably start taping Season 50 before we know it. As I said, if Caroll has plans to retire after Season 50, and with Season 50 coming up soon, then this Farewell Tour of his makes sense.