I personally have not tried changing the password on an existing scheduled task but the TaskScheduleControl() UDF appears to have that functionality.

Borrowing from the preamble:
Quote:

;OVERVIEW A collection of Windows Task Scheduler control functions
;
;REQUIRES JT.EXE - Microsoft Win-2K Resource Kit Tool
; (Available from ftp://ftp.microsoft.com/reskit/win2000/jt.zip)
;
;SUMMARY TASK = "what to run" - program & parameters, security
; TRIGGER = "when to run it" - one or more per task
; EVENT = "task and related triggers"
;
; tsInitTask() Initialize a Task array to defaults/nulls
; tsInitTrigger() Initialize a Trigger array to defaults/nulls
; tsDefineTask() Load a Task array with user data
; tsDefineTrigger() Load a Trigger array with user data
; tsSetCredentials() Define the user ID / password for a task
; tsSetEventCredentials() Update the userID / password of an event



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