First off, thank you for purchasing and supporting ScoreCam!
Unfortunately there is no current path to upgrade from PRO to Subscription. Apple handles all the financials so there is no way for me to give you a refund directly. However, I would suggest you ask for a refund from Apple and then go ahead and purchase the subscription.
Here is the official support link from Apple on how to get a refund: Request a refund for an App Store or iTunes Store purchaseTags: in-app, live streaming, purchase, refund