It's possible to change it.
I'm not sure just how you're supposed to change it, though, and a sym-link doesn't strike me as the way HP would do it. It's been a few years since I've played on HP-UX. There should be a /bin/posix-sh, though, which will definitely be the POSIX shell. In practice, there's very little between them.
Wade.
"All around me are nothing but fakes
Come with me on the biggest fake of all!"