I ran across this article explaining how to delay doing the free windows 10 upgrade of your Win7 - 8.1 PC indefinitely. As you probably know Microsoft has set the deadline for the free upgrade to Windows 10 at July 29, 2016. After that, if you haven't upgraded it will cost you as much as $199 to do it.
The article explains it way better than I ever could. This may still not make you happy because it does involve doing the upgrade and then backing it out, but it will allow you to keep your old Windows for a bit longer if you are wanting to upgrade at some point, but not before July 29.
http://www.zdnet.com/article/how-to-lock-in-your-free-windows-10-upgrade-and-keep-using-your-old-windows-version/
I have already upgrade both my PCs, and don't want to go back, so I can't tell you how it worked for me, but I thought I would share this for those who might want to do it.