Here is the installation procedure for installing Windows 10 from Windows 7 as a free upgrade. It is pretty simple, many people have reported doing upgrades from their legit Windows 7 installation, not accepting keys and detecting a valid installation. Here is screenshots of an installation done from the installer from the Microsoft website.
First we go to Microsoft Windows 10 upgrade site: http://www.microsoft.com/en-us/software-download/windows10
Then we scroll down and select the version of Windows that we want to download. Eiter 32bit or 64bit, most likely you will download the 64bit version.
Right Click and save the upgrade package file to a location where you use it.
After loading the upgrade package this will be the first screen that you see. Select Upgrade and click next.
Now sit back and wait, this will take some time as the install files that are downloaded are well over 3GB in size.
A quick message pops up before going to next screen.
Checks for updates.
We're getting a few things ready...
Now you can type in your product key.
If the key was accepted install will start.
You can can choose to keep your programs and documents, or loose programs and keep documents... or you can blast them all away!
Your computer gets checked to make sure it has enough space for upgrade.
Hit the Install button to start upgrade, and sit back and wait.
This will get up to 100% then the computer will restart, and finish the upgrade.