If you would like to change your subdomain name, it is possible to do so. For example, you can change your subdomain from companyname.joinportal.com to newcompanyname.joinportal.com.
This is not currently supported in the Portal application directly, but you can contact our support team and we will be happy to make the change for you.
Note: This is in the case that you want to change your joinportal.com subdomain, not connecting a custom domain. For information on custom domains see this article: Connecting Custom Domains.