There is a working but feature poor demo available for Poser 5. ProPack is also still aailable. I say yes, upgrade. But you will have a great deal more to learn and a host of new features to deal with. The most significant of the changes will be the materials system -- it's no longer simple, and there are no masters of it at this time.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)