You do not need to wait until it's offered to you, you can already download the ISO, or download/start a utility that'll create a bootable USB stick: (Not that you need the bootable part, you need to start the setup from within W7, as you'll be upgrading...)
See this page: Download Windows 10On that page, a link is shown, to the Installing Windows 10 using the media creation tool
And here's Microsoft's words on the upgrade key stuff: (Some words colored red by me)
"
On the Enter the product key to activate Windows page, enter your product key. The product key should be in a purchase confirmation email if you bought Windows 10. It looks like this: PRODUCT KEY: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX. Note If you upgraded to Windows 10 on this PC by taking advantage of the free upgrade offer and successfully activated Windows 10 on this PC in the past, you won't have a Windows 10 product key, and you can skip the product key page by selecting the Skip button. Your PC will activate online automatically so long as the same edition of Windows 10 was successfully activated on this PC by using the free Windows 10 upgrade offer.
Hm, hold on, maybe you can still do a clean install?
But, like and successfully activated Windows 10 on this PC in the past ... now, what's the deal with that?
I'm lost a bit...
Good Luck!