Just to clarify: you cannot change the main domain (the free subdomain that comes with the account). That used to be allowed, but resulted in stranded, inaccessible sites/accounts when either the user let their domain expire or (as was the case with the CO.CC fiasco) the domain provider ceased service with no notice. You can use .htaccess to redirect to your "canonical" URL, and most CMSs (like WordPress) build their links based on a URL you provide in the configuration utility, so users don't necessarily have to know that the site can be accessed via the free subdomain, but the free subdomain will never go away.