Yes you can do that, simply create a proper hosting account under the primary domain and move your installation there.
To ensure directly linked articles find their new home, I'd recommend setting up a dynamic 301 redirect to seemlessly shift people to the new address. This article will detail how to do it:
http://codegarage.com/blog/2011/03/how- ... a-new-url/Moving Wordpress is pretty easy, mostly you just need to change the 'Site address' in the wordpress settings (which will break the existing installation as soon as you change it), and then move the site files and update the database settings accordingly.
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Mr Kissyfur wrote:
Pretty much everyone agrees with Gnomes,
really, it's just some are too right on to admit it.