You can still activate Windows 10 using a Windows 7 key, e.g. an OEM CoA key that comes with your laptop.
This activation method was officially supported by Microsoft but was meant to be temporary. You can check out these two articles for some background. Apparently Microsoft never really closed this activation channel, probably on purpose.
I would like to make two quick remarks in this post:
This activation method still works as of March 2018.
OEM keys work, too.
I say this because the Thurrott article claimed that it had to be a retail key. I have personally tested a Windows 7 Pro key that is shipped with a Dell Latitude E6540 laptop and successfully activated Windows 10 Pro.
That said, the key only worked when I try to activate Windows under Settings/Activation after Windows 10 has been installed. When I entered the key during the Windows 10 installation process, the key was refused. To me it’s just a small annoyance. You just need to make sure you select the correct version of Windows 10 that corresponds to your Windows 7 key during the installation. The mapping table can be found here.
Below I have attached a series of screenshots when I went through this process on a Dell Latitude E6540.
Can I still use the key for Windows 7 after the key is used to activate a copy of Windows 10?
Yes. See here.