The main domain must be an "x10" subdomain.
If you wish to change the main domain to another x10 subdomain, please:
1. Make sure you are posting from the Forum Account linked to the Free Hosting Account in question
2. Check to see that your choice is not currently in use
3. Post it here so an Admin can see it
4. I would remove any subdomains/email accounts, etc attached to the old domain. They can be hard to remove once the parent domain has been changed.