How does Windows 10 activation work(Free upgrade)?

[Dobbsjr]

When you upgrade is your previous Windows 7/8 key free for use on another PC? 

[Speedbird]

When you upgrade is your previous Windows 7/8 key free for use on another PC? 

Yes it is. Despite the myths about the key being 'recycled', you can still use it on another PC.

[GoodBytes]

When you upgrade is your previous Windows 7/8 key free for use on another PC? 

If you upgrade your system to Windows 10, your Windows 7 and 8 keys is considered invalid as per the license agreement you agree upon when you upgrade your Windows (this applies to all version of Windows upgrades). Trying to circumvent this is against is illegal, and therefore not permitted to discuss on this forum.