How does iOS change file system format during upgrade to iOS 10.3? During upgrade to iOS 10.3, the file system format will change from HFS+ to Apple File System (APFS).

How can the process retain the user data when changing file system format (how reliable it is? sounds risky)? Is the process reversible (when I try to downgrade back to iOS 10.2)?
It's long been possible to convert one filesystem to another on the fly - Paragon NTFS-HFS Converter is one of many utilities that can handle the task.  

Equivalent technology could readily be applied to a new disk format - though best guess would be it is a one-way process right now &amp; to revert you would need to wipe &amp; start over.

I wouldn't dream of doing either without a solid, tested backup in place.