I think that powershell is supposed to give you the ESD file name...you should have it on your computer. Im pretty sure its downloaded before its used.
I think when you download update on your activated system and create iso from esd file so it is activated. My computer is activated from my esd update. I clean install . Good!
Findings: Upgrade from 8.1 PRO > 10240 PRO (Activated) Generic key after install! Upgrade from 7 PRO > 10166 PRO > 10240 PRO (Activated) Generic key after install! Upgrade from 10166 PRO > 10240 PRO (Activated) Generic key after install! Clean install 10240 PRO (Unactivated, wont activate) NOW THE INTERESTING BIT! Windows 8.1 PRO > 10240 PRO (Activated) Generic key! > Fresh 10240 PRO install (Activated) It's still using the generic key. But unlike the first time i clean installed this time it did activate. (Because on that same machine it had 10240 activated prior) From this i conclude once you have upgraded anything activated to 10240 and that's activated, you can then fresh install using the generic 10240 serial and it will be activated when you hit the desktop! It does seam to tie activation status to your MSA. I am a little confused by the fact whatever i do the serial is the same! Hope this helps.