@Crystal-MSFT - thanks for the screen above. After MC288489 device can't be locked down to a specific user anymore. If everyone can sign-in, option "Hide change account option" in Windows AutoPilot portal doesn't make any sense. It does nothing. It doesn't prevent the sign-in of a non-assigned user on the device.
So I would expect - Microsoft should communicate the change according to the importance of the "downgrade" and never without notice. It's a commercial service. We all pay for it. @Jason Sandys