Well, the idea is not that it's a password, but the idea is that it's an encryption key. The key is part of the encryption process.
Otherwise it would be possible that somebody copied your messagebase, stored (part of) it in another configuration of TB and read it, but now even the folks at Ritlabs can't read your messagebase without your password.

So in order to change your password you've got to unencrypt your message base (create a backup), delete all links to your old password (uninstall and install again) and encrypt your message base (restore backup) to your new password (encryption key).

Actually it isn't necessary to uninstall TB, it's also possible to simply delete the message base |(after creating the backup) delete all TB entries from the registry (everything in the tree:
HKEY_CURRENT_USER\Software\RIT\The Bat!
)
But as most people don't like to edit the registry manually, the suggested action is uninstalling.