I am about to buy PD11 but here's my questions. I already have Windows 8.1 (Package) If I buy PD11 and install Win8, is it possible to upgrade from 8.1 to windows 10 for free? or do I need to purchase Win 10 license from Microsoft store?
Yes, if you have Windows 8.1 valid license key (and it's not an Enterprise one) you may install Parallels Desktop 11 and use option from screenshot. Except for that please check this KB for more information.