Mac storage basics · Disks, APFS and formats
Can I convert a Mac OS Extended drive to APFS without erasing it?
Short answer: Usually — in Disk Utility, select the Mac OS Extended volume and choose Edit → Convert to APFS. Back up first; it can't be undone and exFAT or NTFS drives must be erased instead.
Often, yes. In Disk Utility, select the Mac OS Extended volume and choose Edit → Convert to APFS. The conversion keeps the files, but back up first anyway.
It can't be undone without erasing, and it isn't offered for every drive (for example older-style Time Machine backup drives). Converting from exFAT, FAT or NTFS always requires erasing.