YES AND NO,
The key you have for W7, and for 8/8.1, (providing its genuine) WILL give you the update for free,
and is required to update to W10.
BUT you >>only<< have 30 Days to downgrade back to your original OS,(W7/8), if you dont like it.
Therefor after the 30 days "you will lose the key" and be permenantly on W10.
You do however have until 29 July >>2016<< to decide
Roy