upgrading from 7 premium to 7 professional

when i first started reading all the info on win7 one thing that i read was that all the 7 versions were on the install disc and that you got a licence for the one that you paid for such as 7 home premium and if later on i wanted to upgrade to 7 professional all i had to do was pay whatever the cost of the upgrade was and they give you a licence key for that new upgrade and bam, there it was already installed and ready to go. does anyone know if that is the case or does one have to buy a completly new version to upgrade from 7 home premium to 7 professional??
thanks

Yep, in some cases you can use "Windows 7 anytime upgrade" to change versions - but it depends on the installation media you have.

This Wikipedia article sums it up well : http://en.wikipedia.org/wiki/Windows_Anytime_Upgrade

how does one know when they buy win 7 that it has this anytime upgrade built in?

From Wikipedia: See image below from Newegg's website:





As you can see; it will cost you $84.99 to do the "Anytime Upgrade" from Home Premium to Professional.