How to redirect from one yola-managed custom domain to another.

I would like to register a new custom domain name with Yola and then redirect traffic from an existing Yola custom domain to the new one. The tutorial here: http://www.yola.com/tutorials/article...
says to "Click on "Assign my domain name to a new site" under Advanced Settings, but there is no such option to click on (I have other custom domains through Yola, so although I have not yet purchased the new one I want, I assume the Advanced Settings options I see now are the same I would see after purchasing the new one).
The options available are "remove" the current primary domain, "add an existing domain", and "add a new domain". If I select "add an existing domain" I get an error/warning saying "Unable to remove primary domain. Since this is the primary domain of one of your other sites, you cannot remove the domain from this site" - which doesn't make a lot of sense because I am trying to add, not remove. Choosing "add new domain" leads me into the process of buying a new custom domain. If I purchase new domain there, will I get the chance to set it as primary and existing domain as secondary?
Could someone detail how to accomplish this seemingly simple goal?