I want to convert "large town" to "wooden castle", how do I do that?

I'm trying to make the cities to convert to castles with the same kind of wall and vice-versa. It seems I've succeeded in part, because the result is ultimately what I want. However, the problem is that the building is called "Convert to Castle" when it should have been "Convert to Wooden Castle", and it shows the image of a Castle in the information box (there where you see details about the building you want to build, how it looks, what bonuses it gives, etc.), not of a wooden Castle. And on the campaign map it shows a castle (the 3d image of the castle), while on information about the building, I see a wooden castle.

Can you please tell me how I can make a correct change things in this matter? (to transform a city with wooden wall to wooden castle and vice verse)