Hi @anparkinson,
More details of the new PID-controller can be found here: PID control
At the end of the first post you will find a conversion tool to translate the old PID’s to new PID’s.
PID converter
If I remember correctly there were a lot of other major changes from pre-AP3.9 versions.
I would be very careful just “cherry-pick” a few well understood param’s for the new plane.