Been using Win 10, for months, one morning the wife is screaming can't login.
Toshiba laptop was displaying invalid email address as User.
Computer was originally installed with Local User Account under Win 8.

I plugged in an Emergency Boot usb, ran System Restore.
The local account sign on was restored.
After recover I created a new local user as backup, with admin role.

Guess what, couple of days later the Start Button of original local user stopped working,
I was able to use backup user, and problem went away.

Friend just drop off her laptop, with invalid Microsoft Account as user.
Tried restoring System Restore back one prior update, no luck.
The local user files are still on computer, so is local account as displayed by
wmic useraccount still show all accounts on computer.

How do i set or change user display by Winlogon??