I have an Office 365 E3 instance that was set up by someone else. Unfortunately, we have created users with the default domain name (e.g. email@example.com). I now want to set up a custom domain (e.g. bluecorp.com) but there is a message in the O365 admin centre: "Otherwise, you'll need to update your users' email addresses when you connect your domain."
Can someone describe how this might work? After I set up the custom domain, will I be able to log into O365 admin centre using firstname.lastname@example.org or will I have to use email@example.com? However this account won't exist in our tenancy so will I effectively lock myself out?
Any help would be greatly appreciated!