Yes you can change a forwarding account to a mailbox at any time. And vice versa.
Just log in to your account via http://www.xxxxxxxxx.com/customerlogin.asp and choose "Change Account Type".
PLEASE NOTE: Replace the "xxxxxxxxx" with the name of your email address, for example starfleet.com.
Please Note: If you have a forwarding account you will not be able to log in to webmail and you will not be able to send email using your email address.
A forwarding account only forwards email sent to your email address to an email address of your choosing.