How does iOS change file system format during upgrade to iOS 10.3?
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.