I'd be grateful to know if there is a way of upgrading from Z15 to 16 without the full system reinstallation and needing to back up settings and files?I'm guessing not but asking.
Not at this time. There are rumors from Zoringroup that they hope to have that feature released later in the year.
You'll probably never be able to upgrade from a beta. From 15.3 to 16, as others have said will happen in the future. At the moment you cannot upgrade from Ubuntu 20.xx to 21.04, due to a bug.
This is so Sad