If you already have purchased the old Business Calendar Pro, you just have to keep it installed on your device to activate all the pro features of Business Calendar 2. Unfortunately we have no possibility to automatically check, if you are a pro user of BC 1. We can only check, if the old BC Pro app currently is installed on your device.
If you don't own the old BC Pro or don't want to keep the old app installed, you can simply activate the pro features of Business Calendar 2 within the app using in-app payment. You will find more detailed information here.