I've got a process set up in Process Builder that updates fields on custom object A when one of its child records from object B changes. My problem comes into play when B is re-parented to a different record of A. How can I both edit values on the previous A record as well as the new A record?

I know I can accomplish this through Apex, I'm wondering if it's possible to do through Process Builder?

1- Diff. between controller and Apex class?

2- any specific reason to use custome controller?


Can anyone help to understand these two concepts?

Thanks!