I don't believe Stardock has the ability for you to pay the difference. Please request a refund here.
Stardock Support Options - Account Mgt. or Purchase/Refunds
https://stardock.atlassian.net/servicedesk/customer/portal/4/group/-1
Choose "Refunds" and describe your problem.
Supply all the documentation you can.
Make note of your ticket number and don't resubmit as it will kick you to the back of the queue.
Then purchase the Pro version from the Stardock site.
NOTE: Please remember this is the Holiday season and employees are taking time off. Also, the Northeastern United States, where Stardock is located, are experiencing a severe Winter storm.