Our security people wanted us to change the passwords on the accounts we use for the replication service, sis, and netinstall service, so i thought it would be no big deal.
Wrong. I spent Saturday on the phone with Netsupport and they graciously saved my ass.
First of all, don't change the password on the netinstall service account. Create another account. In the Netinstall Manager, tell Netinstall to use the new account. Wait several days (or weeks) until you think all your computers have had a chance to update. Then and only then, disable your original account.
Thanks again to Andrew, Holger and the gang for helping me out!