There are many questions regarding Windows 10 that so far seem like they can only be answered by experimentation, by giving whatever it is you're interested in a try. I was curious about what would happen if I tried upgrading a regular Windows 10 Home 64 install [in a VM], that itself was an upgrade from Windows 7 Home Premium 64, to the new Insider Build 10565 now that an ISO had been released. I fired up that win10 VM, ran setup from the ISO, & with 2 hiccups it worked. I don't know if or how posting the results might be useful, but in the interest of adding to the public knowledge re: win10, here they are...
The way you're supposed to do it is associate win10 with a Microsoft account when you install it, though that is optional. Then you can sign in to that account & join the Insider program, & then install whatever preview build, again associating it with that same account. In that copy of 10's Advanced Update settings you select either the Fast or Slow ring -- Fast gives you more frequent upgrades -- but in either ring when an upgrade is pushed out you cannot ignore it. You're supposed to be able to pause or stop your participation in the Insider program, leaving the installed build intact.
I didn't want this copy of 10 associated with Any account, nor did I want it to be part of the Insider program. *Maybe* because there was never a Microsoft account associated with this copy of Windows, win10 setup froze when it got to the screen where you enter your Microsoft account data -- I had to reset [hard restart] 3 times before it would let me click Skip & proceed.
The 2nd problem I expect will be an occasional annoyance... Because it was never set up with the Insider program, the part of the advanced upgrade settings where you select fast or slow rings simply says that it needs fixed, & in the half hour that I was running win10, Windows told me that it needed fixed a couple of times. The button is there to stop Insider builds, but it wouldn't work -- I don't know if that's a bug, or if because it was never set up for the Insider program, it can't leave the program.
My original questions were how the 1st ISO since 10's RTM handled Windows' keys, Microsoft accounts, activation [particularly on a VHD for a VM], and potential drawbacks to upgrading a regular build to a Insider preview. The 2 problems noted were the only issues I encountered, so I consider those questions answered [using BTW the most expendable of my win10 installs]. I personally consider this sort of thing risky...
*If* Insider builds continue to have hard coded expiration dates, which they may or may not, does stopping the updated builds leave you with a copy of win10 that will expire? If your copy of 10 is tied to a Microsoft account, & maybe if it isn't, *If* something goes wrong & you lose activation, can you get it back? Will the upcoming major update work with the preview builds, & if so, will the result be the same as regular win10 installs. If not, will there be a preview build that achieves parity, e.g. the prior Insider preview ISO was essentially the RTM?
microsoft.com/en-us/software-download/windowsinsiderpreviewiso