this is something theoretical way of getting administrative access to the machine you are looking to change the preferences.
Assuming it is a Multiple Users password, boot into any live OS. Go into System then Preferences, locate the file named Multi-User Prefs and throw that file away. Then do a normal restart.
Be warned I have not tried this so i cannot guarantee it. However it should delete all the accounts on the drive, allowing you to use the administrator account to change the IP.
If you still have the MAC OS 9 disc you can try the "Archive and Install" option which is essentially like a recovery install, however it will create a new System Preferences, allowing you to login as an administrator.
Hope that helps