Changing Keychain Passwords via command line/terminal

I'm trying to unlock and then change the password for the login keychain from the command line, but can't seem to find a way to do it. I've flipped through the man pages for security and systemkeychain but it seems it's only possible to add/delete items, rather than change the password of the keychain itself. Can anyone give me any insight into how I might do this? Many thanks. John