On CO.com you can see the balance change in almost real-time by applying a Regional upgrade to a reservation with R availability (the balance of upgrades available goes down by one) or changing a flight with an upgrade already applied to a flight with no upgrade space avalable, which in effect removes the upgrade (and causes the balance to go up by one). I discovered this by chance two weeks ago when changing a reservation for a traveling companion who I had upgraded as a special treat.