Actually, Poser 7, like Poser 5 supports multiple runtimes, so I think we are pretty safe.  I just upgraded to Poser 6 (the get P6 free with your pre-order of P7 deal) and I just left Poser 5 alone on a separate drive and linked to it's Runtime.  It's worked great.

My P5 Runtime has passed 21 GBs, by the way!

I have this noble idea that some day i will make different runtimes in catagories, one for each character and one for props and such of different genres.  But, when will i ever have the tiem to sort through all the stuff!? lol.

One other note, when I upgraded to P5, it left P4 on my system, When I upgraded to P6 it left P5 alone, and I suspect it will be the same with P7.  I'm backing everything up onto a separate drive just incase, anyway.