Because they are just placeholders it's done on a hardware id when you upgraded.Stored on M$ servers the only time you need a key is when it is clean installed on a machine that never had it installed before.You can install without a key and phone them to activate if you change motherboards for example but be prepared to spend some time on the phone mine took over an hour.
The activation screen will give you the # in your area and you will need to have that up when you call.It'll be self explanatory as you try to activate.