According to Apple’s rules it’s not possible to cross-grade to a free world version or upgrade to a higher edition outside of the App Store. Sorry.
How do I purchase an upgrade?
To upgrade an existing license please log into your account and choose the license you’d like to upgrade. Your user account is tied to the email address that you used for purchasing. Request a temporary password here. You’ll be sent a link that allows you to enter a new password for your account.