The upgrade is non-destructive-you won’t lose any files. If that happened to you, there’s a fix to upgrade to APFS after your installation is done. I’ve seen several instances in Apple’s forum where people forgot to check the box before a hurried click on the Install button. What’s mildly tricky here is that the description of APFS, at the bottom in the smaller print, is isolated from the checkbox above the disk icon. High Sierra (Beta) installer with checkbox for APFS. Because Apple has had stunning success with APFS in iOS so far, you’ll probably want to upgrade your Mac to APFS, especially if your boot drive is Flash-based. When the public beta of macOS High Sierra is released, you’ll likely have the option to stay with the old HFS+ or elect to upgrade your boot drive to APFS. APFS will enable lots of new technologies on your Mac.

For reference, here’s a quick overview of the APFS features. APFS is a modern file system that replaces the extraordinarily aged HFS+, introduced in 1998. One of the notable features of macOS High Sierra is the introduction of the APFS file system.

High sierra macbook air early 2015