Hi,@CM123
Thanks for posting your question in the Microsoft Q&A forum.
Based on your description, I need to confirm a few questions:
- Did you log in with the modified account? Can you provide a screenshot of the error message?
- Does Microsoft Authenticator (MFA) have any alternative verification methods, such as email or phone number? If so, have you tried using other methods to verify?
According to your question, I conducted the following test:
1.I changed the username of my Exchange account.
- I can log in successfully using the new username.
Under normal circumstances, you can log in after the modification.
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.