Although they can be upgraded, the mechanics are different. Some buildings can be upgraded without the upgrade kit, for example the Confectionary and the Tree of Love. Shrink Kits are in essence like upgrade kits however they only reduce the size of the buildings and do not increase the production properties.
Note that the upgraded building will be from your current era, even if the old one was from one of the earlier ages. Buildings that become larger in size will be stored, while buildings that keep the same size or become smaller will stay in your city. After the upgrade kit has been used, some buildings will be stored in the inventory and must be placed in the city again manually others will be placed directly on the same spot in your city, and the production timer will not be reset.
To upgrade a building, the upgrade kit must be used on the corresponding building that is already situated in the city. Upgraded buildings gain better properties with each level. The Upgrade Kit is an item that can be used to upgrade an eligible special building to a higher level.
