I have started experimenting with patrician vassals but they pay only half of city taxes and can barely even make trade posts at the early game. I like to make church dukes to rule my realm because they are much easier to please then feudal lords AND pay taxes(If you are christian you have to make them like you more then the pope but usually with free investiture and minimial church levies they do). Your new vassal will make a vassal of the correct type to hold the one he isn't supposed to have now. Step2: now give this same new vassal the county capital, it will auto switch the holdings around so the proper type is on top. Remember the ORDER he is given the titles is everything.

Step1: give the SUB holding of the type you want to be the new capital to any character with no holdings at all, he doesn't need to be highborn or royalty or anything, all that matters is the order in which he gains land, he must be landless or already the type of ruler you want the county to be. To do this revoke the city / barony / church from the sub holding owner after you get the capital of the county, or build the proper one and sit on the province for two years. Switching a county is fairly simple, you must own the county capital(the county) and a sub-holding in this same county that is the type you would like to actually be the capital.

Ck2 wrong type of holding