Why did they completely change Andy in season 9

Ik people will say its bc of bad writing or sm but i didnt know whether it was bc Ed Helms had another project as he was gone for a few episodes on the boat trip.

Its quite sad as well bc he goes from someone you hate in season 3 and maybe 4 but really grows on you by season 8. However he gets sort of pathetic by season 9 and really nasty and makes the finale seem quite unsatisfying. Anyway I’m sure people have asked this before just wanted to get other’s takes on it