It looks like a regression, in fact it should be like that
at least until 1.9.0.beta10 +12 version I’m using.
EDIT : I can see the Start Upgrading button again, only if the upgrade fails for some reason. In this case it reappears. Check the log to see you find errors or not.
EDIT 2 : I can confirm that after the upgrade (done now) the Upgrading and the Reset Upgrade buttons are no longer displayed and the Start upgrading remains visible and clickable even if the upgrade is running.