For example, on one Macbook, I have Mavericks. If I consider the Catalina too new and might not be too stable yet, can I choose and upgrade to either Mojave or High Sierra?
(App store won't show search result for Mojave or High Sierra. I had to use a link and open in Google Chrome to get to either Mojave or High Sierra in the app store.)
The real question is, can I upgrade from one MacOS to another freely? That's because, if there are 7 versions that are commonly used by people (Mavericks, Yosemite, El Capitan, Sierra, High Sierra, Mojave, Catalina), then upgrading from one version to any one is "7 choose 2" and therefore there are 21 possibilities, and is that supported?