Have Windows OEM Product Key -but key is not accepted when Installing New Windows 8.1 Disk

Question

I was able to secure the Product Key for what I thought was a Windows 8.1 OS on my Toshiba notebook.  I believe my notebook ORIGINALLY had an OEM version of 8.0 (was pre-loaded when I bought the notebook) but at some point I updated the OS to 8.1.  I
was able to verify that I was currently using 8.1 via the Bios set-up utility.  After securing the product key, I contacted Microsoft and they agreed to send me 32 and 64 bit versions of Window 8.1 free of charge (so as long as I covered the shipping.)
When I went to install the  8.1 disk, I received a message stating that the product key didn't work.  I can't help but wonder IF perhaps because my PC originally came with 8.0 that it's product key won't work with 8.1?  When Microsoft asked
WHICH version I had on my PC, I had advised that I was using 8.1. They never asked what was the ORIGINAL OS?
Any ideas?

Answer 1

thanks for replies.  I suppose it would be helpful for me to explain why I needed the product key and the new Microsoft Windows 8.1 discs in the first place.  This was never a matter of merely trying to activate the OS.  Last week- my
notebook went in to standby mode- a very normal process when I would step away from the notebook for a decent amount of time.  When I tried touched the power button to release it from stand by, I received a message that the OS was not able to start up
due to TWO (very specific) files that were either missing or had been corrupted.  The message recommended re-installing the OS in order to re-insalll or otherwise correct the file errors.  I spent a good 3-4 hours researching this on a number of
Microsoft/ IT sites, and it appeared this was a very common error -stemming from "boot disk error" -or "errors with hard drive/ partitions".  I then began my trek to a) locate the product key (of which I was able to secure from the BIOS) and b) have Microsoft
send me the Windows 8.1 discs.  Hope this helps and thanks again.