alt Hacker NewsYou can but you have to tie it to actual devices and a point in time, not simply a specific OS version. Essentially, all devices that existed before the change must still support the old set of characters and devices produced (or sold or activated) afterwards can support the reduced set.
Or wait until a future OS version that will not support any device currently in existence.
This fails if they let you keep your password migrating between devices, though, so you probably need a version somewhere in the middle that flags it as an issue and flags it as not allowing migration without changing the passphrase.