You can still upgrade an activated Win7 installation to Win10 for free. Just run the setup from the Win7 desktop and don't enter any key--it should activate itself after connecting to the internet. Once Win10 is activated on that machine, then you can perform as many clean installs as you like afterwards forevermore and each time it will activate itself because the activation info is stored in the cloud.
Thing is, if you have an OEM machine (like Dell, HP, Lenovo, etc) then Win7 was originally activated with a SLIC table meaning your CD-Key on the sticker may never have been used so you should avoid wasting it, by doing an upgrade install first. Then you could use it in the future to install Windows 10 on another machine later, even one that isn't Windows 7 compatible.