Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hello, remh
Microsoft accounts allow you to create new @outlook email address and add existing email address as aliases, which you can then use to log in to other software, such as Office applications (Microsoft 365 Family subscription).
If you already have another personalized email address, you can refer to this article to add this personalized email address as an alias for your Microsoft account: Add or remove an email alias in Outlook.com - Microsoft Support
However, you cannot directly modify the original personalized email address or directly create a personalized email address using the above steps.
Best wishes,
Lucia Wu - MSFT | Microsoft Community Support Specialist