Solaris

how did you change the shell , by editting the /etc/passwd file right? did you make any typo? also ensure that /bin/bash is inside the /etc/shells file (if exists) but not necessary

This is Solaris. Has it got bash?
You are well advised to not change the shell for "root" on any system.
On the rare occasions when you need to edit a password file use "vipw" then check afterwards with "pwck".
On systems with a shadow file you may need to run "pwconv" to synchronise shadow after a manual edit, but do
check what is correct for your system first.
The "vipw" command validates the "root" entry and will not let you save an invalid shell.

Of Course GOT bash

Instead of Editing the /etc/passwd file you can use this command which make it reflect immediate

# passwd -e <username>
# old shell >
# new shell
# confirm shell

If it require apply pwconv which refresh the passwd and shadow file