Thanks for all your input, guys. I found a very simple solution to my problem. Remember that the main objective here was to test DNF Automatic, so I needed to downgrade any random package from my server installation to create the need for an update. I simply chose a package from a third-party repository whose policy it is to keep older versions, and I chose Lynis for this:
# dnf downgrade lynis
This installed version 3.0.8 instead of 3.0.9, and I could happily test my automatic upgrade script.
Problem solved. Cheers !
Niki