reverting from Catalina to earlier OS

I have a late 27" imac with a 3TB internal drive. It currently is divided into three partitions.. one is running catalina, the second is a carbon copy of the first - the third is my music library. I would like to revert back to the system before catalina - I just don't like it. I understand that the partition holding the main startup system has been changed from HFS+ to something else , kinda dividing it into two. I have plenty of external space, including the previous last-saved setup. My questions are as follows... Can I simply reformat the main partition into the previous setup and carbon copy my old saved setup.. and then use the Carbon Copy backup of the catalina system to boot from, ikn order to update prefs etc?? Do I need to reformat both partitions - or do I need to copy my music partition and just reformat the entire interior hard drive? I AM franticly backing everything up as I go... but just need a little guidance on the "AFS to HFS+ thing...