In my current game, I switched alot of cities to wealth, for three reasons:
1. I had just ended my Golden Age. I spent my entire Golden Age stockpiling Cossacks - to the point where I was spending 50 gpt on military costs as a Monarchy. When the Golden Age ended, I went from making 80 gpt to losing 80 gpt. I needed wealth, bad, and a war. So I set my cities to wealth, and took over the Dutch. After I finished them I just barely had gained enough cities and lost enough Cossacks to not need to spend any gpt on military upkeep. (I went from 70 Cossacks to 50, and gained about eight cities.)
2. The reason why I was making so much during my Golden Age as opposed to adding it to research was twofold. First, I already had a significant tech lead and was still making good time - I could research new techs entering the Industrial Age in eight turns. Second, I was going straight for Replaceable Parts, forgoing everything else, so I could skip from Musketmen to Infantry and from Cannons to Artillery. I knew I would need alot of money to upgrade them all, even with Leo's, so I started stockpiling all the money I could find. Even after I got Replaceable Parts, I still needed extra money to finish the Cannon to Artillery transfer, so I kept trying to make money.
3. Most of my cities were done building improvements, and as you already knew, I definitely did not need any more military. The only improvements my cities could still use was Colosseums - I never build those - and Cathedrals. (Only in the small cities - the large ones had Cathedrals, since I had the Sistine.)
So, I turned my cities to wealth to turn the deficit into a surplus until I could finish the Cannon to Artillery transfer and until I researched Industrialization so I could start building Factories.