How do I change shells (bash to ksh) on OS 10.9.1?
The easiest way is to just start Terminal as usual and type ksh to start a Korn shell. If you want to open a new Terminal window for it, type Shift-Cmd-N and run ksh from there.

Depending on how your .profile looks like, you might need to change it as well to identify the shell it's running under and execute initialization commands accordingly.