The official help (Upgrade Zorin OS - Zorin Help) talks about saving all your data, then installing the new version over the old one, then reinstalling all your additional programs.
Is this because it's thought that typical users wouldn't be able to cope with doing it the 'apt-get update && apt-get upgrade && apt-get dist-upgrade' command line way, or has Zorin broken that? (I know that Linux Mint did do so for a significant period, which is why I stopped using it.)
Or is there, in fact, a GUI way to do in place upgrades between major versions?
No, as @Storm mentioned, an in-place Direct Upgrade is in development. Mint is a larger team that the ZorinGroup of Two... But all the repositories must be set up with packages in place for the Direct Upgrade to properly function and to not break the O.S. by sourcing the package from the wrong repository.
The ZorinGroup has stated that the Direct Upgrade development is currently their High Priority Item.
Well, it takes some doing to break it - it's been utterly reliable on Debian for two decades and I can't remember a problem with Ubuntu - so I am glad that fixing it is a priority.
If it is fixed, I hope the release announcement makes a big fuss about it.
(I do not have a problem with paying for someone else to do work - what Pro is. But generating lots of work for me via not being able to do direct upgrades is a problem. We can, I hope, disagree whether stopping doing something upstream manages perfectly is breaking it or not, but either way it is so large a problem for me that I can't use Zorin OS.)
Fixing it is not a priority. Creating it is.
It is not Broken in Zorin OS.
Debian and Ubuntu both do not offer a pay-for Pro version.
I am now locking this thread. The question has been answered. There is no need to spread misinformation. Nor do I feel like chasing after a thread that will undoubtedly wish to debate the merits of having a Pay-For Pro version.