You have got lots of things here! If you can, I'd avoid changing the LAN domain as I've seen it end in tears and I am not sure why. If it does end in tears, do a configuration restore.

You need to find somewhere to host your domain. There are plenty of domain providers, including Clearcenter. You'll need one to set up all your DNS records except your PTR record. Moving your domain to another domain host should have no effect on ClearOS.

Do you have an Dynamic IP and what do you want to do with your poweredbyclear.com FQDN? The Clearcenter DDNS can either update your poweredbyclear.com subdomain or your domain, if they host it, but not both. If you ask them nicely, they may convert your poweredbyclear.com subdomain to a CNAME record pointing to your domain.

If you have a static IP without control over your PTR record (your ISP controls this) or a dynamic IP, you will need to relay your e-mails out via either your ISP (if they permit it) or via an SMTP service such as your duocircle.com account, or many e-mail servers or ISPs will either reject or SPAM your e-mails.