I love the concept, but why would you want to?
From my previous summation of the reasons I've seen posted by myself and others.
1) Meet a quest more easily (my personal favorite
).
2) Adjust market conditions by changing what we are receiving from buildings so that surpluses in some goods are lessened.
3) Get the building to produce (or look) the way we wish it to for our city.
4) To adjust space by returning to a smaller building.
5) Lower the cost of tournaments by lowering the number of AW levels. (This may have not been mentioned here, but has been elsewhere).
Comments added here:
1) Quests sometimes ask you to do impossible things -- like raise a buildings level to level X. If all that type of building are at their highest level already you have to find the room to start a new one and build it up from scratch to level X. Finding the room for the quest may mean destroying a building or at least teleporting it. But if the X asked for is only 1 or two levels down from the building you have, and you can downgrade it to that level, problem probably solved (though there may be a shape and size difference, it's usually smaller and thus, easier to place).
2) If there's a surplus of a good you can shift the production of the building to produce something other than what is not needed. This increases the value of your production for trading purposes.
3) Similarly, Gum back to seeds, bismuth back to moonstone and so on means you are producing what you need. Just because you won a building in Chapter X doesn't mean what it gives in Chapter X is what you need. X-1 might be a better fit for your needs.
4) Since space is at a premium a smaller building might be needed to make space for an upgrade of another building. You may not need as many scrolls as your buildings are producing and if by downgrading you get a bit more space, that might help.
5) Since the cost of tournament play is tied to the level of AW's it might be advantageous to be able to lower the cost of tournament play by lowering the number of AW's upgrades. This is probably the most difficult one to contemplate as it might be that players will just lower a whole bunch of non-military AW's to bring their tournament scores up. I'm, personally, not in favor of this one....but it has been mentioned.
Hope that helps with the question.
AJ