You wouldn't lose anything because you could link your P5 runtime to your P7 so your content would still be where you have it now and how you have it set up now.
Poser 5 was the first version of Poser I owned and it isn't in the same league as 6 or 7 IMO!
Also if you have a multithreaded processor or dual core system then Poser 7 makes use of it and renders much better and quicker and handles textures and memory better so no more OOm messages!

Injustice will be avenged.
Cofiwch Dryweryn.