I made a Recovery Drive of Windows 10 14383 from my Latitude 7350 and clean installed it on an OptiPlex 390 system which has never been made a Windows 10 Pro Device:
I then used the OptiPlex 390's Windows 7 OEM Key to activate Windows 10 RS1 Insider Preview.
I have completed the same test with build 14393. There isno audio here as the results are identical:
Notes: As we have everything updated in these Insider builds to match the final release in order to stamp out the last bugs – this includes the license agreement, Edition and lack of watermark. Then we can say for sure that as far as the Operating System is concerned Unused Windows 7 OEM keys are still accepted. Any changes to Product Activation are not incorporated into the installation media but this not rule out changes in the Microsoft Product Activation Server.
My personal opinion is the Free Upgrade will continue after the 29th of July… Windows 7/Windows 8.x keys are too embedded into Windows 10 and the deliberate breakage of product activation without time for testing on an Insider Preview build is a really silly move for Microsoft.