Solved, sort of:
Your reply gave me an idea.
So as root, I first ran
# calibre-uninstall
Then when the uninstall had finished, still as root I ran
# wget -nv -O-
https://download.calibre-ebook.com/linux-installer.sh | sh /dev/stdin version=4.9.1
and this did download and install the right 4.9.1 version.
Now, why this did work I leave to your sagacity ;-3)