After you add a custom domain to Office 365, you might want to change the name for your team site from, for example, contoso.sharepoint.com to fourthcoffee.sharepoint.com. But unfortunately, it isn’t possible to simply change the name or use a custom domain name for a team site because of how the architecture works for Office 365 team sites.
If you must change the name of a team site, you will need to create a new Office 365 account and use the name that you want to use for the onmicrosoft.com domain as part of sign up. Other steps you may need to take include removing your custom domain from the original account (the one with the team site name that you didn't want) and moving any email, files, or other data you may have set up with that account to the new one.
Tip: To move team site content to a new site, follow the steps in this KB article.