Reading the FAQ linked here it appears that once you buy Windows 10 it doesn't have an end of life cut off so carries on indefinitely subject to "Lifetime of the device"
This has to mean that system upgrades will be very restricted and more than likely will be exactly the same as the present OEM license which means that when I upgrade my motherboard and/or CPU I will need to buy another license.
Any other scenario would mean that Microsoft would lose an fair chunk of revenue
This is more than a one-time upgrade: once a Windows device is upgraded to Windows 10, we will continue to keep it current for the supported lifetime of the device – at no additional charge. With Windows 10, the experience will evolve and get even better over time. We’ll deliver new features when they’re ready, not waiting for the next major release. We think of Windows as a Service – in fact, one could reasonably think of Windows in the next couple of years as one of the largest Internet services on the planet.
Edited by Oldjim (Fri 23-Jan-15 14:17:00)