That's the thing: you don't! The charset for passwords should be always inputable even if no one is using it. If you wanted to reduce the size of the charset,

That's the thing: you don't! The charset for passwords should be always inputable even if no one is using it. If you wanted to reduce the size of the charset, you'd basically create a transition plan, and ask everyone in the world with a passcode to set a new passcode and validate that against the new charset/rules. A company that can perfectly transition the world from x86 to ARM can surely manage that.