If you happen to have a Win 10 Home machine you want to update to Win 10 Pro AND you have a spare Win 7 or Win 8 Pro or above disc lying around you can use that serial for a free Win 10 upgrade. Right-click the start menu, choose Settings, go to Update and Security, under upgrade choose I have a Product Key and enter the Win 7 or Win 8 Pro serial and follow the directions to upgrade. Quick and easy :)
 
I'm pretty sure you have to use an install disc that you are not currently running...and must be Pro or above.
 
Bill

I have also heard of legit windows SN's on ebay for like 4 dollars. One dude says to look at the seller ratings and comments. Claims he had decent success. Sounds too good to be true. Might be worth 4$ to find out. The number needs to clear Windows though or I'm out.

I upgraded a Win7 Pro PC to Win10 Pro for free.  So if you already have a Pro edition of an earlier Windows version 7 or 8, the upgrade to Win10 keeps the Pro edition.

Wish I knew that before I paid $199......................

FWIW, I'd avoid eBay offers that appear too-good-to-be-true...
Those are often pirated install codes.
We've seen this happen many times over the years.
 
If you upgrade directly thru Microsoft, you have nothing to worry about.
If you are sight-impaired, I believe you can (still) upgrade to Win10 for free (directly thru Microsoft).

I've bought cheap Win 10 (and Win 7) licenses from Amazon in the past, most which require phone authorisation from Microsoft. I've never had an issue with them though.
 
I have paid attention to reviews/feedback though. Unless it's > 98% (ebay) or mostly 5 star (Amazon), I usually avoid.

msmcleod
I've bought cheap Win 10 (and Win 7) licenses from Amazon in the past, most which require phone authorisation from Microsoft. I've never had an issue with them though.
 
I have paid attention to reviews/feedback though. Unless it's > 98% (ebay) or mostly 5 star (Amazon), I usually avoid.

Same here.  As long as you respect that they are OEM licenses (which means you can't re-use the license in any way) you should be fine.
 
Of course, I typically only activate my O.S. just once, during installation.  After that, I rely on system images as a safety net to restore from any software issues.  If you're having to constantly reactivate the O.S. then you should probably stick to a retail version anyway.

And I have bought cheap shrink wrapped Windows installation discs on eBay that, although legitimate MS software products, had product keys that had already been used to activate them and were rejected by MS activation. Beware of these cheap code deals. The best of them are usually for volume licensed software that were not installed by their original commercial owners. The license is generally not transferrable. If they do activate, you will probably have a working system as JonD says, but it is not all that rare to find that they will not. If they are cheap enough to shrug off the possible loss, then roll the dice.
 

You can legitimately "upgrade" your motherboard/processor/RAM... and the OEM license is still valid.
It just can't be installed on more than a single machine.