Currently there is nothing stopping you building the Kingdom equivalents to the Empire special buildings (the ones that have equivalents to Kingdom).  This is generally considered to be OverPowered (OP).  It is quite possible in the future (and some mods have already approached this idea) that only either 1. you can't build equivalents to special buildings already built, or 2. you will need to convert all buildings (and the ones you don't have an equivalent to will be demolished) and will not be able to use buildings in any major way until the population (or town) is converted.

Best regards,
Steven.

Well, I'm sure there is some good system that the devs can make that both gives you some advantage for taking over a town/city but doesn't allow you to do +25%, +50%, or +100% twice or more.  The simplest way would be to block the equivalents of other alignments of towns (so you'd have the captured type but couldn't build your equivalent) but currently, you can build all your types of buildings in a town that was originally another alignment.  When there are Kingdom/Empire buildings that are functionally equivalent, it is kinda OP to be able to have both in the one town.  But you can do this at the moment (1.09e.049).

Best regards,
Steven.