After writing the above, I saw a bunch of postings about the upgrade path from 11 to 13 and realized it seemed somewhat risky if it was done the right way. Then I realized I could try it on my private computer first (which also has Catalina, and where I have another license of Fusion Pro version 11.)
So I purchased the upgrade at 99 EUR, only to realize that the license key generated only works for Fusion 13 and is not backwards compatible with version 12...
You mentioned it should be down-gradable, @dempson , is there a process for that which I should explore? I opened a support ticket, but the lady that contacted me said the system did not given them the option to generate another key.
Thanks.