Multiple versions of jjOS installed on one machine?

I've read up on jjOS so much lately I can't remember where to look it up again or if I even saw this right e.g. I know you need a free version/specific version to install certain other versions I think, so maybe I'm confusing something like that for this. Is that the only instance in which you have multiple versions of jjOS on a machine?

I'm not asking to bounce back & forth between versions or need to figure out what to install before installing something else, I ask because I came across a 1000 with the free jjOS...but I think it has jjOS2XL on it too?

A screen says "tap tempo + power on -> OS2XL"...that means it has jjOS2XL and you just need to prompt the machine to boot it up next time right? Why else would it say that? Has the owner been letting it auto boot the free version because they don't understand another version is on there and you launch it with a button prompt during power on? I didn't want to say anything and end up increasing the asking price or make the owner look dumb , but I don't know enough about jjOS (yet) to understand what I'm looking at when I see that screen .

So I've been doing some more reading and I believe versions are installed over one another, giving access to both the current version of jjOS installed and whatever the previously installed jjOS version was.

Anyone here had multiple versions of jjOS on their machine because they upgraded/changed from one to another and is able to switch between their current version and the previous one installed? I think machines with free jjOS that upgrade to paid versions retain the free version.

Yes as per the boot you screen you can switch versions with tap tempo. However it might not be an activated paid version, could just be free trial (saving disabled)