I apologize for digging up this old thread, but I was looking for a way to this myself. It seems like this question hasn't been answered in any of the topics, so I just figured out a way to do this and I'd like to share it.
You can edit spiritshifting models in the statuseffects.gamebundle file located in
\PillarsOfEternityII_Data\exported\design\gamedata
I personally chose the cat form, so I have to change the cat related entry. Search for Spiritshift_Cat_SE_ChangeForm (replace cat with your form).
To change into one of the Ranger companions, e.g. a lion, replace the NewFormAppearance entry with "art/character/creatures/lion01/a_lion01.asset".
We've now changed the model, but we still have to attach the proper animations by changing the AnimationController entry to "art/animation/animationcontrollers/creature/lion.controller".
After doing that, search for Spiritshift_Cat_Shifter_SE_ChangeForm and do the exact same thing again.
It's pretty straight forward. The only time consuming task is to dig through 70.000 character entries to try to find the one you want.
