New Company bought old company; Want to add New Company emails to old Active Directory ... Not working

Question

1) So we have a company with an existing server infrastructure and active directory , we got bought by a new company and changed our email addresses with MS365 (still have the old MS365 email addresses also) the old email addresses are how we logged onto all the computers...

Can we update the active directory to include the new email addresses at sign on ?
(i'm not a computer guy really, scientist..., old company got closed then restarted with some former employees)

2) I updated the UPN to have the new email address in AD , but when I try to add a new user it says that user already exists (because the old email accounts are in the AD) How do I add the new company email addresses to the AD ?

Answer 1

O365 is not currently supported here on QnA. The product group for Office actively monitors questions over at
https://techcommunity.microsoft.com/t5/office-365/ct-p/Office365

--please don't forget to Accept as answer if the reply is helpful--

Answer 2

actually switching the UPN did fix the issue it just took a couple hours and then going into the user properties and switching the email and UPN

Answer 3

Hi,
Thanks for your posting here and sharing the resolution!
As you said, after you add the new upn suffix , you can just use the same user name with the new upn suffix as the new email address.
For more information ,you can refer to Match the AD DS UPN with the Microsoft 365 UPN

You can accept the solution you provided as the answer to anyone who encounters similar issues.
Best Regards,