@Nick_Boyts That’s weird, I can change it manually
Oops! I forgot I was looking at a non-workshared project! My bad.
Try changing it for just one reference plane. You could have one in your list that’s being controlled by something else. It works for me.
Interestingly, I was trying this today with a script I made (and ran) successfully before.
It seems not to function with Areas that are in Groups (returns message to the effect that ‘the Parameter is Read Only’).