My business requirements is that users need to able to sign-in to Office 365 using UPN with email address format. Currently, their username is the <EmployeeID>@emailaddressDomain.com.
A customer has Azure ADConnect. Do you have the suggested solution to do it?
will Outlook configuration change?
How about Azure AD Connect and Outlook configuration?
First of all you have to change the UPN in your On-Premises AD, then do a sync.
After you change the UPN like Juan said, your users have to use new UPN format to login and to Outlook reconfigure the profile.
Note: After you change your UPN services like Skype for Business their login and SIP will change to the new one.