Expect script to su to root and perform sed

You will have to do something like this:

#!/usr/bin/expect

set timeout 15
set ssh_server servername
spawn ssh $ssh_server
  expect {
       "User*:" {
           send "UserID\r"
           expect {
               "Password:" {
                   send "passwd\r"
                   
                   return 0
               }
           }
         }
}

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This is not complete.  You will have to follow this logic and have to test at each step because it is hard to program it without having your complete setup.

Follow examples like
http://floppsie.comp.glam.ac.uk/Glamorgan/gaius/scripting/5.html
http://www.thegeekstuff.com/2010/10/expect-examples/

I would say you are going about it the wrong way.

You should be using ssh keys and proper tools to modify /etc/passwd

For example:

#!/bin/bash
for host in $(cat /path/to/hosts.txt)
do
  ssh root@$host "usermod -c 'Some text' username"
done

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1. For security concern, most of companies doesn't not allow ssh root@<remote machine>.
root access is either through console login or "su".
So I thought of security requirement and ruled out ssh root@host.

2. For changing the default shell for multiple users, replace string on /etc/passwd is quicker than usermod command.

For my expect script:
-----------
...
# Replace strings
send -- "sed 's/TEXT/NEWTEXT/g' /etc/passwd > /etc/passwd.new\n"
--------
can be replaced as
--------
send --"/usr/bin/perl -i.bak -pe 's/TEXT/NEWTEXT/g' etc/passwd\n"
-----
Which will generate backup the old /etc/passwd as /etc/passwd.bak

Perfect, exactly what I was looking for.