Unable to assign governor to a previous holding planet that is now class 24

I used a prospector ship to claim several class 0 planets as holdings in government type oligarchy.  As the game progressed, some of these class 0 planets were upgraded to class 24.  However, they remain as holdings and i am unable to find a way to assign a governor to these now class 24 planets. I think, once they become a class 24 planet, they should stop being a holding or at least have some way of assigning a governor to them.