As the title says, for multiple people I know, iOS 10.3 forced us all to change unlock passcodes, even those of us who recently changed it. On top of that, there is now a minimum length for passcodes - 4 characters.
Just an FYI for anybody who didn't see this in the update notes (I didn't).
Just an FYI for anybody who didn't see this in the update notes (I didn't).