I don't know, it seems to me they could easily offer upgraded culture buildings that are identical in footprint, with a minor art variation.
The most difficult part would be ensuring that their price was competitive for people who didn't have the previous version, while programming a mechanism to get partial credit for the earlier version.
Perhaps buying a diamond-priced culture building sets a flag, and only people with that flag set can buy the upgraded version.
Or, selling a diamond-building puts a certificate in your bag of holding that can be consumed as a kind of credit toward the higher version.
Or, they just need to make the buildings up-gradable, with the cost being in diamonds, which doesn't seem like it would be difficult. All they have to do is set the "upgrade" materials to be something that doesn't exist in game (Adamantium), with a price to obtain them via diamonds.*
Which would also allow them a way to add something to the game that you can normally only get via diamonds, so that people could obtain adamantium, which they can use to upgrade diamond buildings.