So what happen if I upgraded my Macbook Pro to High Sierra? Will the SSD be converted to APFS on the fly? Or does APFS only apply to first time clean install? I want the pros of APFS, but I don't like clean installing the OS on my Mac as it is a day-to-day tool so I cannot really afford to spend too much time clean installing and restoring my data. As far as I know, one cannot just switch file system on the fly, so I'm wondering what magic Apple is going to do.