Someone beat me to the above! B-5 was, and remains, my favorite show ever.
Isn’t that from Voyage of the Dawn Treader? When did that one ever get a TV adaptation?
I think the entire cast of BtVS changed for the worst during Seasons 6 and 7. Since I missed the last half of Season 6 (but I read up on them at TWoP, which was enough) and watched Season 7 mostly out of a sense of duty I can’t dredge up the details, but all the characters lost some of their humanity and the whole thing turned into one big fanfic, what with everyone kicking Buffy out of the house after everyone had skipped town (hurt!) and then Spike sniffing her out several hours later and cuddling up next to her (comfort!). This after he tried to rape her, mind, which a couple of seasons earlier would have got him dusted like that.
For the few of you who are going to get this reference, imagine Cassandra Claire writing in the Buffy fandom and not the Harry Potter fandom.
I always thought being caught drinking the colonel’s liquor while smoking a cigar at Blake’s desk was WHY he only drank grape soda afterwards. 
No one’s mentioned Bill Bixby yet, the most obvious “change”. And weekly, already. And yes, he does fit the OP, or at least the title.
It’s officially in the series continuity: clearly eight regenerated from seven (who was shown), and there were a few 8th Doctor radio adventures.
In Naruto, Sasuke turned from cool, to kind of useless.
I don’t think Costanza changed that much, if we exclude the pilot.
Daria became much less self-assured
The BBC had a series of Narnia adaptations in the late 80’s, however the quality varied from adequate to dreadful. Although you did get to see Tom Baker as Puddleglum, which almost redeemed the whole enterprise. They’re out on DVD, for Lewis completists.