Presumably there is a known mapping of the old vs new values. It would be friendly for Poser to include a script to adjust a figure's materials accordingly. "Convert P11 glossiness to P12" or something like that.

Right now, what I know is that whatever values I have are wrong. I don't, however, necessarily know what they need to be changed to. So I have to check my materials and see whether they have the relevant nodes, and then do a bunch of trial and error to correct them. That's still not optimal.