I don’t quite understand what you mean by saying you want to “use a new domain” and “not use it”. However, all Dreamhost accounts now have unlimited domains, so you are always free to add new domains to your account either in addition to or in place of the one(s) you already have. You must, however, register all domains separately, and once you’ve used the one free domain registration you get from Dreamhost, you must pay for any further domains, though you may register them with other registrars and still host them at DH if you wish. If you really don’t want your original domain, you can set it up not to autorenew the registration, and then it will eventually expire (do this only if you don’t mind somebody else grabbing it, and possibly putting porn or something on it). If this happens with regard to your free DH registration, they’ll probably let you regster a new one free after that.
You can also create unlimited subdomains of any domain you have hosted at DH, and those don’t require any registration fees.