If you are on the fence about upgrading and not quite there yet, you can follow the instructions on the following web pages to reserve a free upgrade for later use. It will take some work, but will save you the cost of a licensed disk in the near future.
- How To Geek article
- Infoworld article (by Woody Leonhard)
The least favorable option is to upgrade to Windows 10, then immediately perform a rollback to your previous OS. The reason I say this is less favorable is because I have done this twice and had serious problems after rolling back, both times.
These methods all work if you actually manage to load Windows 10 and log in at least once. This is because Microsoft uploads a special file from your computer to its upgrade servers. This file is a fingerprint of your motherboard and CPU and acts as a digital entitlement to a free license. Formatting your hard drive doesn't delete the digital license as it is stored in the MS Cloud.