Change default domain - impact?

Question

We acquired several small companies in the past years and put them together as custom domain names in our Azure tenant, sharing the same level. Now I was asked to check the impact of setting one of the domains as default. We use Duo Single Sign-On for Microsoft 365, a few users have MS Dynamic assigned and all our users are Office 365 active users. What might happen if I set one of the domains as default?

Thanks!

Answer 1

The existing accounts arent affected, but any new accounts you create will have the default Azure domain set as their domain suffix in their UPN

https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/domains-manage#set-the-primary-domain-name-for-your-azure-ad-organization