She sells C shells on the seashore!
Anyway, back to it. I hadn't noticed before that you seem to be using su root rather than su - root. The latter construct picks up root's .profile and any other variables appropriate to root, the first won't. To set the shell variable once logged in as root, you can probably do an export SHELL=<path to ksh>. That won't necessarily change your shell, however, but you can issue a ksh to do that. Let us know how you get on.