As you already read from the googleGoogle support pages:
at this time you can't change which domain is your primary domain.
You can't do it. So there is no way of "keeping" your free googleGoogle apps plan if you insist on changing the primary domain. However, keeping the primary domain is not usually a problem if all you want to do is change the "Organization name". You can change the Organization name under Domain Settings -> GeneralDomain Settings -> General
. Once you add the new domain name to the account you can also log in at google.com/a/newdomainname.comgoogle.com/a/newdomainname.com
.