Can I Upgrade and Only Pay the Difference?
You can upgrade any time.
Because MemberPress is an annual subscription, the cost of the upgrade is pro-rated. So the sooner you upgrade, the more will be applied towards it.
Upgrading is simple in MemberPress. If you decide that you'd like to upgrade from the current version you are on, simply log in here, go to the Subscriptions tab, and then to the far right of your Membership, click the Change Plan link.
That will send you to the standard MemberPress purchase page but the prices you'll see are prorated based on what you've paid already, and how long you have had that membership for. So, the sooner you upgrade, the more you'll be credited.
After clicking the Get Started button on the edition you would like to now have, you will be taken through the basic purchase process again.
For more details on the upgrading process, please visit this page.