Hi DK422,
Thank you for posting in the Microsoft Community Forums.
Impact on PC users: If users have gotten used to using the old SamAccount login name, the change to UPN will require them to use the new login name to log into their computers. This may cause some brief confusion or inconvenience, but once users get used to the new login name, this effect will disappear.
Continued use of SSO: If the SSO system has been configured to support the new UPN login name and the user has updated their login information, they should be able to continue to use SSO to access relevant applications and services. However, if the SSO system has not been updated or configured incorrectly, users may need to manually update their login information or contact their system administrator for assistance.
Best regards
Neuvi