"Microsoft is yet to explain the exact details of how the free Windows 10 upgrade will work for users of Windows 7/8.1."
"While that info is expected as we get closer to Windows 10 RTM, which is currently sometime this summer"
That being said I assume we will get an installation key to install the Windows 10 upgrade, if so you could use this to reinstall W10 at a later date, it is not clear yet is if you have to have a previous Windows version installed before you can upgrade to W10.
Edit: here is the new windows 10 FAQ page from Microsoft
Although I can find no Microsoft statement I believe the following is true, once you install the free upgrade to Windows 10 use software to recover the Windows 10 product key from the registry and store it in a safe place, then if you ever need to reinstall it after one year you will have the needed product key, but will still need an installed, activated qualifying product to upgrade to Windows 10 again.