How To Find Old License Key?

I moved away from using Apple computers in 2015 and I'm not actually reverting back to Apple. I originally purchased Parallels Desktop 9 in 2013 and then upgraded in 2014 with v10.0. I can't find my old license key but do I have the original order receipts for both the v9 and v10 purchase.
How can I go about retrieving my license key? My original Parallels Desktop account was closed/deleted when I no longer had use for it. I believe I can pay the upgrade price to upgrade my v10 to v15 instead of paying the full price? Any assistance would be greatly appreciated.
Thanks

 

Hi, do you have any purchase orders? or email which you used for the Parallels Desktop purchase?