Catalina Upgrade

How to convert HFS+ to APFS: https://www.macobserver.com/tips/how-to/upgrade-mac-apfs-after-high-sierra/

Usual: Have a backup prior to the conversion. Definitely have a backup prior to the Catalina upgrade. This backup is your only path out, if the Catalina upgrade should fail, or should Catalina fail to work for your requirements. Confirm your important apps are all 64-bit. No 32-bit apps will work with Catalina. Confirm your printer and scanner vendors have drivers available.

Starting with Mojave, this is no longer optional for SSD based drives.

You will need to backup your information and convert it to APFS with Disk Utility.