You are making a classic mistake.
You shouldn't use the username and password in the session data.
This is a security risk and serves no purpose.
All you need is a flag in the session-data that says 'loggedin'
The only way to get that flag set in the session is to let the user log in using his username and password. That is the only time the password is used.
When the user logs off, you set the flag to 'notlogged in'.
Now when a user changes the password, nothing need to change in the session at all.