User onpromise AD Domain -> Azure AD Domain change

@scott, chae While we would love to get more information around this to be able to guide you thoroughly, from the description that you have mentioned it seems like you are trying to onboard AAD now.

If you can help us with your need like - if you just want users to synchronize to AAD or you would want to implement Azure AD Domain Services As well.
If you need some information about How Azure AD Domain Service can help you, you can read about it here.

Either way you will have to synchronize the users to AAD, you can follow this guide to see best practices and implementation guidance. If you have any questions related to AAD Connect, you might find is answered here.

Coming to the example that you mentioned :
Onpromise AD: userA@hi.com -> Azure AD: userAA@Hello .com

So lets say, If you own hello.com you can verify that domain under your AAD tenant, so that the users who are getting synchronized from your local AD has that domain.
For that, you also need to add a UPN suffix in your local AD with hello.com and change the UPN of the users to hello.com

As as per current setup, hi.com will result in changing the UPN from hi.com to the default tenant domain, something like abc.onmicrosoft.com unless you change the UPN on prem.

If the user is synchronized from on prem all users attributes are synced to the correspinding AAD account automatically, which can then be used by other services like Sharepoint or office 365. You can also read about how does other SAAS services like Sharepoint uses the user profiles of that user which gets synchronized to AAD here and implement their information

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You should use AAD Connect - https://video2.skills-academy.com/en-us/azure/active-directory/hybrid/whatis-azure-ad-connect
So you can keep your AD still on prem (and manage it there) and it all get's synced to AAD.

If you have schema additions - you may have to configure them additionally

best regards
stephan