If I buy the 48 hour rental can I upgrade to the Digital Download Copy by just paying the difference?
Yes, you sure can. Since the 48 hour rental lasts for 48 hours, you have 48 hours to upgrade. You upgrade by clicking the 'upgrade to digital' button in you 48 hour rental area.
If you've already logged out, you can upgrade by logging in with your email and the password your created and then once logged in, you click on your 'store' at the top. The option to upgrade will be there. Here is where you login: