I loved watching Sherlock but then the last few series I found beyond awful and ridiculous! Gone was (imo) the clever writing and semi-believable show.
Also found shopaholic series got overly rubbish towards the end. Same with the I heart book series. Saying that, I know they're not award-winning literature to start with.
Forget changing your likes and dislikes. What series of books/films/tv shows/music did you like until.. it changed?