We set "Hide" as value for "Hide change account option". The result is: any user from the org can put any UPN in the sign-in screen and enrol the device.
Before MC288489 the experience was: only user assigned to the device in the Windows AutoPilot was able to enrol the device. No option to change account was present on the screen - see attached.
If this is the result of the MC288489 - then Microsoft please: 1. Never do it without notice in advance (3 months+) 2. Correct the interface in MEM accordingly 3. Correct the docs accordingly on end-date 4. Communicate the impact honestly & properly