After countless attempts to upgrade i did a clean install of Win10 by mistake.
Well actually i did a install NOT keeping any files.
I didn't delete old Windows ,and it created a Windows.old folder.
I was not sure if that is a clean install - i hoped MS means clean install is formatting the Windows partition / or e.g. a new SSD drive.
Well after finishing install (Well finally i saw Win10 in my Dell after the thousand failed upgrade attempts) i was asked to activate and cross my fingers.
And of course i came with the error that the KEY IS BLOCKED.
i then wiped the harddisk and installed Win8.1 , luckily the key is still valid for 8.1 and got activated again.
My question is that (since i cannot do the upgrade at this time, i hope something-happened-fixed in some days/months , after spending a week on it i give up for now :( ):
Can the key be used in Windows 10 in a latter time , or maybe is blocked on Win10 only?