It's been a month and 3 days since I upgraded my internet computer Win 7 to Win 10 and I just dual booted to Win 7 and back again to Win 10, and it's fine.
In the MS support area I see that they say after the 30 days if you want to even reinstall Win 7 from scratch all you need is the product code (or key?). And you can build an install DVD from the Media creation tool
I'm sure that's really nice for those that bought pre-packed computers with no Windows install DVD. This should be encouraging to anyone on the fence about upgrading - it's a no lose proposition and this free upgrade won't be available after July.
Very soon I'll upgrade my DAW Win 8.1 to Win 10.