Many businesses that use Microsoft 365 go through changes that result in the branding and company name evolving. These changes can include acquisitions, mergers, demergers as well as brands and companies changing their names.

When this happens, the tenant name chosen when setting up your original Office 365 will not be appropriate. Also, the original tenant name itself cannot be changed. This is then visible in various places (primarily URLs for OneDrive and SharePoint) to end users and contacts you share information with.