Ordering a premium hosting account itself does nothing to your free hosting account. But once the account is setup, you can request that iFastNet migrate your website to free hosting, which includes:
Updating the IP address of your domain name in the free hosting nameservers to point to your premium account.
Forwarding traffic from the free hosting IP address to your premium account.
However, if you are using a custom domain name right now, you should still update the nameservers of your domain name to point to the premium servers to be able to use all premium hosting features, such as full DNS control.