Performing the free upgrade will get you a key, you may need to use some utility (google can help there) to find out what it is. Based on my experience, machines upgraded from oem pre-installed win 7/8 get one of a handful of generic keys, where as retail copies of 7/8 that get upgraded get unique keys.
You can create Windows 10 installation media for Home and Pro (but not Enterprise) from this page
http://windows.microsoft.com/en-gb/windows-10/media-creation-tool-install
and use the key you have from the upgrade to install it.
