I don’t remember where I heard this: The Catalina install does something (reformat, divide into pieces for a disk ‘group’) to the installation disk. If true, does this make reverting to Mojave difficult or impossible?
Yes, upgrading to Catalina splits your drive into two partitions, one <mydrive> is still the name of the boot partition, and is read-only.
The second partition is named <mydrive> - Data, and has all your files/documents, and apps. It is read-write.
Unless you are certain of what you have - don't upgrade to Catalina without doing a full back up first.
Then, it would be simple to revert if you don't like Catalina. Just erase the drive, and restore from your backup.
My main issue with Catalina is the couple of apps that I use for work, waiting for 64-bit updates. Can't upgrade permanently until I sort that out (or the app publisher does that 64-bit thing )