I might've accidentally stumbled upon something that could get Untangle to reveal the Upgrade button....
I went and checked out Config>Upgrade but it still showed "No upgrades available". I did it 4 or 5 times for good measure... still with the same results.
I then went casually to Config>About and walked through the Server tab (took a few seconds to populate) and then the Licenses tab. When I went back to Config>Upgrade, the Upgrade button was there.
I don't know if my going through those motions actually did it, but it's probably worth a try if you're itching for the Upgrade button to come out.
And oh, my update went smoothly.