Upgrade key for VP10 for Win10

In attempting to install my upgrade copy of VP 10, I managed to get the stallationg program to run in compatibility mode. I entered the S/N fine. It asked for the VP8 CD (which I have). Then it asked for an "Upgrade key." Tried the S/N but it doesn't work.

What, pray tell, is an upgrade key and how does one get one?


Cecilieaux