I have 2 domains that are registered at GoDaddy :

domaina.com (not hosted, only domain name is registered to GD)
domainb.com (hosted at a different webhost, domain name registered to GD)

domainb.com is an already working site, with a different webhost, but the domain name is registered to GoDaddy(and I assume the nameservers are changed to redirect to the webhost).

Now, I don't understand why this was done, but domainb.com is considered a subdomain on the host... meaning the files are in a seperate folder on the server. Ex : public-html/domainb.com/public-html/FILES

The structure is similar to this on the webhost :

HostNAME (main root folder)
    domainb.com (subdomain of hostname)
    domainc.com (etc...)
    domaind.com (etc...)

I want to transfer the site domainb.com to domaina.com, meaning domaina.com will become the new website, without having to re-upload all the content and CMS. The old one will redirect to domaina.com once the transfer is done (using 301 redirects).

Can anyone tell me how I can do this?