Problem solved, i will copy answer from Pocus from another forum, in case someone else will need help (it works if playing on single player): Access the local files of the game with Steam (right click on the game name, properties, local files. Go to the directory: \Scenarios\e550BCGrandCampaign\DATA...

Hi, Playing now as Carthage in DLC 550BC campaign. It starts as 1st level civ Monarchy. When I move to 2nd level civ it changes automatically to Republic. The problem is that it doesn't change capital building. I still have King's Palace in Carthage region, which after few turns becomes Abandoned Pa...