Changing version of Unity

Hey! I don’t know is it only me facing this issue - I changed the version of Unity from Unity 2018.1.8f1 to Unity 2019.4.10f1. So when the files is finally converted, most of my assets and scripts can’t work. One of the assets includes TextMesh Pro. For the scripting, I am using C#.
I realised that the animation that I am using is affected as well. The asset for the animation is Opsive Ultimate Character Controller.
Can anyone tell me, what should after converting the version of Unity, like scripts and animation?

Unity recommends making a backup before changing versions. I’ve never had this happen to me, but I’ve also never upgraded to a version that much newer than what I had (ex: I have done 2019.4 → 2020.1)