The ability to change your Microsoft 365 tenant name attached to .onmicrosoft.com domain is rolling out now. If you have less than 1000 SharePoint sites in your tenant, you should be able to do this today. Microsoft will gradually enable this feature in all tenants in the coming weeks.
This feature is quite handy for scenarios such as:
- Typos in tenant name
- Organisation rebranding
- Mergers and acquisitions
Check this Microsoft Docs article to know more about the implementation steps, limitations and actions you need to perform after changing the name in your Microsoft 365 tenant.
As part of changing the domain name in your Microsoft 365 tenant or signing up new Microsoft 365, you can use the web service at https://o365.rocks to check the tenant name availability.