Before Windows 10 the procedure was that you would run Windows anytime upgrade and either buy a new key code from Microsoft or you could buy a key code from a retailer and just enter the new key when Windows anytime upgrade asks you to. Windows will then unlock the Pro features, reboot and you then have the same installation as before but now it is Windows 7 Pro. All applications and data remains unchanged. The entire procedure takes 10 minutes.
How do you know if the code being sold has not already been activated? I guess it will cost you $25 to find out. Note there is no reversal unless you make a full disk backup if after you upgrade you get a message saying this copy of Windows is not genuine.
I would think if the code has been activated it won't activate again but it may if it isn't being used anywhere else. No different to you reinstalling on a different machine because of hardware failure but your original licence may need to be a retail pack rather that an OEM pack.
Licenses are managed differently with 10 but I'm like you and want to keep 7 Pro as some of my hardware won't work under 10. I tried it and went back. I always recommend the Pro version whether you need the features right away or not as one day you just may. I wish 7 Pro was still available too for a new install but I understand 10 is now needed to support some of the newer mainboards and graphics cards.