I am using Zorin 15.3/64 (free version). According to Zorin's own dox I ought to be able to use an upgrade option from the system tools, but it isn't there. How come?
Welcome to the forums!
I believe, if memory serves, 15 may have needed you to go and install it through the terminal? Or it may possibly be through the software centre now, I would have to check later. Either way, you should be able to install it even though it's not available automatically.
Although note, I believe you're going to have to upgrade from 15 to 16, and then 16 to 17.
ALSO: Please backup everything before going through with it. It will probably be fine, but you never know.
EDIT: I think the package is called "zorin-upgrade"? Someone else will have to verify though, can only do so much at my windows work computer 
Thank you. I am aware having to go through 16. Data is on separate disks, so no probleme there. But will I have to reinstall all software?
So I've never gone through the upgrader, I've always preferred fresh installs. However, my understanding is that you should not need to reinstall all your software.
However, you may wish to ask someone else who's gone through the upgrade to make sure that's correct 
If you erase the linux partition, as most suggest one should do while instaling from usb-stick, it seems to me that all information on installed software is erased too. Please correct me if I am wrong.
On the other hand, the command that is missing from nearly all tutorials, I just found out, is:
> sudo apt install zorin-os-upgrader
Then the upgrader-utility appears in the System tool menu and it becomes possible at least to follow the upgrade procedure.
Thanks to this video.
Yes, if you were going to do a fresh install of Zorin, it would delete all your data on your partitions you currently have.
If you use the zorin upgrader that you now have installed (yay!) then that should not be the case. However, that's why we always say backup anything before doing it anyway. Just in case something goes wrong and things become broken.
You shouldn't need to install all Your software new. But when You have a lot of Stuff installed it could be that maybe something in the Upgrade Process cause an Issue - it don't have to, to say that clear, but it can.
Another Point are added Repo's When You have added third-party Repo's there it could give Issues. So, You should try to deactive then or even delete and add them later again to avoid Problems with that.
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The upgrade procedure stopped with an error status, yet seems to have worked all right. It just may have been the final software update that failed on an long obsolete installation of notepadqq. Now see if I can make the next upgrade to v17...
--- (two hours later) ---
succesfully upgraded from 15 via 16 to 17.1
Thanks for your support.
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Kudos to you for going through the procedure.
Nice to know it works 
And does all works or are there any Issues?