Reinstalling the operating system after upgrading to Windows 10 from Windows 8.1?

[Windows Central Question]

So I'm currently running Windows 8.1 Pro on my machine that I have a product key for and an installation disk. Let's say I upgrade to Windows 10 Pro once it becomes available and use it for a few months, then decide I want to reinstall my operating system for whatever reason. So my question is, how will that work with the Windows 10 upgrade? Would I have to install Windows 8.1 first then upgrade again? What happens if I reinstall my operating system after a year Windows 10 is released, when it won't be a free upgrade. Can I download a Windows 10 ISO file then use my Windows 8.1 product key for it? I'm just curious how this will all work since I tend to reinstall my operating system at least once a year.