Is it possible to in-place upgrade the Windows Server OS on the server when Plesk is installed?
Such scenario is not yet supported by Plesk. The correct work of Plesk is not guaranteed. It should be done at your own risk.
Instead, it is recommended to migrate Plesk to another server running a newer Windows version by using Migration and Transfer Manager.
Please take part in our product improvement by referring to the following link: In-place dist-upgrade windows servers. The top-ranked suggestions are likely to be included in the next versions of Plesk.