Hi ARanebo,
I understand that you want to release your domain from the old account.
To remove a domain from Office 365, please do the following steps.
1. Go to the Domains page in Office 365 by doing the following.
On the Admin page, under Domains, click Manage your website and email domains.
2. On the Domains page, select the domain name that you want to remove, and then click Remove domain.
3. On the Remove domain page, click Yes.
In general, if some user accounts, sign-in address or website address are still using this domain, you will not able to remove this domain from the current Office 365 tenant. Therefore, please double check if some user accounts, Lync sign-in address or website address (sure it’s http://domain.sharepoint.com) are still associated with this domain. Please also check if your administrator account has proxy address.
After that, you can wait couple hours (1-2 hours) for the provision and try to remove the domain again.
More information about how to remove a domain, please refer to the following article:
http://office.microsoft.com/en-us/office365-suite-help/remove-a-domain-HA102818535.aspx
Thanks,
Jason Jiang