It should convert the Home key to Pro (I assume you are talking about the $100 option from within Windows 10).
I was going to recommend the PC World license thread in hot deals, since the Pro license was only $40, but it looks like that promotion has finally ended (the page it was on is completely gone).
When I was considering going that route with my license, from what I read on Microsoft's site, it converts your existing key into a Pro license. They should also send a purchase receipt to your email address, and if they changed it to a new key for whatever reason, the new key would be in that purchase confirmation.Yes, I'm talking about the $100 option within Windows 10 for upgrading to Pro from Home.