Re: We will tell them it's free - Muuhahahha
You can upgrade, for free, from Windows 7, 8 or 8.1 within the first year of release. After those first 12 months, you need to pay to upgrade. Either way, once you're upgraded it's yours and you are supported. That is, it *doesn't* need license renewal - it is not a subscription.
Take at look at http://blogs.windows.com/bloggingwindows/2015/01/21/the-next-generation-of-windows-windows-10/. The key quote is "This is more than a one-time upgrade: once a Windows device is upgraded to Windows 10, we will continue to keep it current for the supported lifetime of the device – at no cost."