IIRC, if you upgrade, you only have one license still, if most EULAs I've read are any indication (Microsoft, Novell, etc are sticklers for that sort of thing.) That's why I bought the whole enchilada (at the academic price of $200) - I have multiple machines, and that way I can play with one while the other is rendering or otherwise bogged down with some foolish modelling experiment I'm working on. As it is right now, I can only have one on. (I wonder if I could do the render farm thang with P4 and P5 in P4 render mode...)