Q:

Is it possible to upgrade tribal holdings of vassals?

I am a new player of CK2 (current patch) playing as the feudal King of Sweden on the standard start date. I conquered some tribal holdings in the North via holy war and, noticing the 'wrong type of holding' penalty, granted these to some of my middle sons and other relatives.
These tribal vassals now hold a negative opinion of me due to 'wrong government type' and contribute minimal levies to me due to the tribal vs. feudal mechanics. I have since figured out the 'upgrade tribal holdings to castles' decision that can be implemented after upgrading hillforts to level 4 and now try to do this for any new tribes I conquer before giving them away.
My question is, if I upgrade my existing tribal vassals' hillforts to level 4 for them, will the 'upgrade tribal holdings' decision upgrade these counties as well, or does it just work for my demesne holdings? This would erase the strong negative opinion modifier and grant me actual levies from these counties, so I think it's worth the expense if possible.

A:

Yes, you can upgrade a tribal vassal's fort for them at your own expense. I did exactly that for some tribal Muslims I absorbed in Arabia just today. Your vassals will become feudal shortly after the stone fort IV completes construction. Their brand new castle is small so don't expect much at first, but if you have cash to spare it's worth while. 

