The example I can think of is a building that produces goods of x age. Recently, some lucky people have been getting palace sets. The Maharaja's Palace produces 5 goods of the same age as the Palace. So if you have a PE Palace, you get PE goods. With this kit you could turn it into a IA Palace and receive IA goods.
The extra part is that while you would down it to a IA goods maker, you would still require the PE population for it.
Later if you wanted to go back to a PE Palace, the pop requirement would stay as is.
If it's then upped again, the pop requirement would raise accordingly.
Edit: So you would pay tavern silver and a high pop requirement to make lower age goods in a smaller footprint than a regular lower age goods building.