It is possible to swap around your primary domain for an add on domain, however to do this you would need to drop us a support ticket with the request.
When swapping them around, it is important to note that any content that you have for the add-on domain will need to be moved from the sub-folder that it is currently in, to be inside the public_html. This also works for the content that you ave currently in the public_html that you would like moved to an add-on domain. Because of this, we would always recommend making a backup of all your current content before asking us to change the Primary domain for you.
Once the Primary domain has been changed, you will need to add the old primary domain (if you are still going to use it) as an add-on domain, and then place your contents inside the folder it creates inside the public_html.