To determine which filesystem is installed on the disk, while in Installer, open Terminal From “Utilities” menu and type the following:

mount

A large list of virtual disks will be presented. You have to find “/Volumes/name_of_your_partition” (for example /Volumes/MacintoshHD or /Volumes/mydisk) in on the list and check the first word in brackets in that line. In this case the partition is simply called “HFS” and is mounted as “/Volumes/HFS”. “HFS” here is not the type of filesystem, it’s just a name of the partition. The thing you are looking for is the first word in the brackets: lowercase “hfs”. In this case the filesystem used is HFS+.

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How to upgrade macOS to High Sierra without filesystem change (HFS+ to APFS)

How to upgrade macOS to High Sierra without filesystem change (HFS+ to APFS)