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.
