if you want to change the password of SP farm admin in MOSS 2007, you can follow this support kb:http://support.microsoft.com/kb/934838/en-us
This kb could also solve the issue while upgrating the farm credential:There was an error encrypting or decrypting credentials. Either a credential update is currently being performed, or you must update the farm account credentials on this server before you can perform this task.
Hope this could be helpful to you
