For many, this could be the best reason for upgrading early. Even when it doesn’t go quite as expected, this really isn’t difficult.
Disk Utility
Volume layouts are fairly straightforward until you have two or more bootable systems with Volume Groups. Details and advice for those with macOS 10.15 or 11.
Disk structure diagrammed for comparison, from the partition table, to HFS+ and APFS volumes, with further references.
What you need to know about the APFS file system, its containers and volumes, so you can understand errors and how to repair them.
What you need to know about the HFS+ file system, so you can understand error reports and how to repair it using Disk Utility and other tools.
What you need to know about disk formatting using the GUID Partition Table (GPT) to understand its problems and how to repair it.
Apple says that if you run First Aid on a disk, each of its volumes is checked too. Is that really true?
Disk Utility can’t image APFS volumes, and normally returns an error if you try to image a container. Here’s the workaround.
If something goes wrong with Catalina’s boot Volume Group, it seems there’s no way to repair it, or change the Data volume. Or is there?
Older versions of Catalina can fail to erase the System and Data volumes, but 10.15.6 should be able to, making clean re-installation simpler.
The Eclectic Light Company 