Well, took a backup and decided to go for it. You don't get a choice, it converts the boot filesystem to APFS. Took about 3 hours on a very full 1.5 TB Fusion D

Well, took a backup and decided to go for it. You don't get a choice, it converts the boot filesystem to APFS. Took about 3 hours on a very full 1.5 TB Fusion Drive, but converted successfully. We'll see how it goes.