The basic process is that when you perform an upgrade to Windows 10 (over a genuine Windows 7 or 8.1 system), an anonymous and unique hardware hash is generated that is based on your systems hardware configuration.
Since it is anonymous, you do not have to use a Microsoft Account.
I proceeded with the Windows 10 installation and anywhere it asked me for a product key I selected the option to continue.
You will go through the normal Out of the Box Experience (OOBE) through this to set some of your options including connecting to a network, using a Microsoft Accounts, setting a PIN and a few other items.
Just keep to the same edition and architecture as I mentioned in my second point earlier.
Before you read this, keep the following four things in mind.
From this point forward any future installs, including one where you delete all partitions and install Windows 10 from scratch, will be activated because of that unique hardware hash and the corresponding certificate.
Since it is all stored on Microsoft’s servers there is no reason for us to keep a backup either.
Your unique hardware hash has been created so in the future, if you have to reinstall Windows 10, you do not need to go backwards to Windows 7 or 8.1 and then to Windows 10 to be activated.
Those of you interested in a clean install onto clean hard drives keep reading: Right after upgrading these same systems, I grabbed my Windows 10 installation media USB drive and I performed the ultimate in clean installs on each of them by removing all of the system drive partitions.