I run 4 operating system drives on my desktop computer, and found out that accepting the upgrade to 10, on one of those drives does not affect the activation of earlier versions on other drives. I use a drive switch panel to keep everything isolated.
For those considering Apple, the product replacement cycle is accelerated in comparison to Windows. Be aware that they update their operating systems approximately annually. They only support the last two versions. So you will be forced to upgrade to maintain a supported operating system. At some point, Apple decides to not offer the upgrade to certain older hardware configurations.
My husband's MacBook was forced into this unsupported status after only five years. The hardware still functions but because the OS is forced into obsolescence, an increasing number of programs were out of reach. This forced him to buy a new Macbook, even though the old one was fine mechanically.
Windows operating system support is predefined and for Win7 extends to January 14, 2020.
Apple offers no time frame to support of their operating systems, meaning your forced to replace relatively modern, functioning hardware (forced obsolescence) at their whim. That's not a commentary on the product, but it chafes if your someone who takes pride in keeping your tools in good shape and functional for the long term.