Basically the VM images we provide are only shortening the setup process in terms of that most of the Debian part of the installation is already executed. I can see no more risks installing UCS from the ISO to using the qcow-image.
During the upgrade of UCS several services will be restarted automatically. At least during this time there will be a disruption. The time of the outage depends mostly on the amount of user and group information as well as on the history of the system.
You may also want to read Barcamp 2018 UCS release upgrades - best practise
hth,
Dirk