Microsoft has stated that Windows 10 will be free to upgrade for the following valid licenses:
Windows 7, Windows 8 and Windows 8.1, for both Home Premium and Professional versions.
However the free upgrade is only available for 365 days from 29th July 2015. So while there is no rush, you should bear in mind that there is actually a time limit on this offer. We are still advocating that people hold off upgrading at the moment due to undocumented features & various issues relating to data integrity.
Once you upgrade to Windows 10, you only have a month to roll back to Windows 7 or 8 if you decide Windows 10 is not for you.
So if you do upgrade to Windows 10, make sure that you explore it fully within a month. After that there is no way back to Windows 7 or Windows 8 except via a full Windows rebuild.
Windows 10 Time Limits Summary
Free Windows 10 upgrade is 12 months from 29th July 2015
The roll back option is one month from date of initial Windows 10 update
Windows 10 will receive security updates until 14.10.2025