With most SiteHQ accounts it is possible to add additional domains/domain aliases to your account via your control panel. This domain 'sits' on top of your original domain and can be given to your users instead of the original domain.
If you wish to change the base (root) domain name associated with your account this will entail the deletion of the original account and the creation of a new account using the new domain. There will be an administration charge for this.
Please submit a support ticket for more information including your new domain name and the account username.