iOS 10.3 will change your iPhone, iPad, and iPod's file system – don't worry, your data is safe!
Thankfully, when the time comes, your data won't be erased in the process. Somehow, Apple has managed to preserve your data from being wiped in the name of a pain-free user experience. However, creating a backup of your data is still highly recommended in case anything goes wrong.
It is unlikely that the switch to APFS will enable major new functionality for users. It could, however, improve device performance thanks to its optimization for solid-state drive storage.
via iPhone Hacks