Will my NT licences Transfer to Win 2000

I am adding 4 new Windows 2000 servers to my network. I currently have 50 NT 4.0 (per server) licences on my PDC. If I change them to per seat will that transfer to Windows 2000 so that 50 users can access any server on the network or will I have topurchase Windows 2000 licences? What will happen to the licence status if I upgrade my NT PDC later to Windows 2000?