I have heard from a Win10 beta tester that it will not let you go back to your old operating system once it's installed. I don't know if this is true, but it would fit with Microbrain's business model. No dual boot either. If you get Win10, it sounds like you are stuck with it. If, and that's a very BIG if, I install Win10, it will be on a dedicated W10 system and not anywhere near either of my daily use 'puters. Agzin, I have no idea if this is true, but I for one am not going to chance it until the general public starts reporting on it "in service".
Steve
I have Windows 10 technical preview installed as dual boot with Windows 8.1.
There is always the option to either update the existing OS, in which case,
with the preview, the process is not reversible, or perform a clean install, in
which case dual boot is perfectly possible.
The offer is Windows 10 for current owners of Vista, Windows 7 and Windows 8.
It is not obligatory and is free for the first year after the release of Windows 10.
This does not mean the if you take advantage of the free offer, you will have to
pay after a year, it does mean that if you acquire Windows 10 within a year of its
release, you will not have to pay for it.
This appears to be an extension of the offer of Windows 8 for £24.99 for a limited
period after its release.
I hope this dispels some of the nonsense being posted in various places.