I installed W10 and then i reinstalled w7
The way I understand it....
IS, that once a machine has Windows 10 installed and properly activated then, that machine is good to have Windows 10 re-installed, for the life of the machine, no product key required (Microsoft maintains that information with respect to that physical machine and Windows 10).
AS to whether or not you, after going back from Windows 10 (upgrade) to Windows 7, can.... at a date, after July 28th "upgrade" that same machine "again" to Windows 10, for "free" I do not know.
Logic might suggest that, you should be able to since the machine already has had Windows 10 on it and the installation has been recorded in the annals of time, but....
There may be a caveat that defies logic, regarding the cutoff date for the "free" upgrade.
I think a better question would be.....
Why, with more than a week left to still take advantage of the free upgrade would you even want to cross that threshold?
My guess would be that you'd be OK, but boy do I often guess wrong.
There are just to many factors that could come into play and since no one knows with any certainty what metrics Microsoft uses to grant your computer its' "Digital Entitlement" and what would make it distinct from the thousands of other computers sold by the same manufacturer that are "exactly" the "same", one might assume that the process is very sophisticated involving unique hardware IDs, but.....
Might it also include the presence of third party software? Would you then be upgrading the "exact" same machine that you had previously "upgraded"?
AND... on or before August 2nd, I suspect that there won't be any handy access to installation media for 10240 or 10586 since Threshold will be upgrading to Redstone 14xxx
Why poke the bear.