We frequently see many forum posts with users of older machines that took advantage of the Windows 10 offer,
The posts mostly focus on the need for a new hard disk as the OEM disk had failed and the OEM recovery is lost as well. Hard disks are reliable but they eventually fail as they have moving parts. Backup your files now.
This is not a problem, any machine that has previously had Windows 10 already installed on it can be activated with a clean install on a new disk.
What is happening is that Microsoft licenses OEM upgrades with a digital license. This is then attached to your Microsoft account which is part of the overall licensing model Microsoft is now using. Microsoft now recognizes that some homes may have several machines that tend to accumulate.
When Windows 10 asks for a key, this can be skipped and when you connect to the internet the machine will activate. Windows 10 recognizes the machine and it will be recognized fine and activated when you sign back in with your Microsoft account..
This also means you can upgrade your disk if desired. Windows 10 will install fine.
Many are replacing old disks with a new SSD, which can perk up the performance of older machines. SSD products are more expensive so there is a trade-off between capacity and speed.