If you bought your Mac with Sierra or earlier operating system, when you boot your Mac with command+option+R key combination, the Recovery Mode and Installer for Sierra are being downloaded. It seems that Apple didn’t think all things through and haven’t updated the images of Internet Recovery Mode, which you can automatically download and run in case of local Recovery Mode being erased. I did some benchmarks on the speed differences between HFS+ and APFS filesystems, should you want to you know what differences may be expected from the change. I’m not saying which filesystem is a better choice. This howto (hownotto) only shows how to keep HFS+ filesystem during macOS High Sierra upgrade. This is not a “you should”, it’s a “you can” It is however still possible to avoid filesystem conversion and install High Sierra on HFS+ filesystem, or keep HFS+.
With the final (17A365) version of High Sierra, such question is no longer displayed to people with systems on flash-only storage. During the installation of developer and beta versions of macOS High Sierra, user was asked if he/she wants to “upgrade file system to APFS”.
