I think ... you could modify the scheduled notification (Instance of UILocalNotification) ...
Here is from the Apple documentation ..
Once you have created an instance of UILocalNotification, you schedule
it using one of two methods of the UIApplication class:
scheduleLocalNotification: or presentLocalNotificationNow:. The former
method use the fire date to schedule delivery; the latter method
presents the notification immediately, regardless of the value of
fireDate. You can cancel specific or all local notifications by
calling cancelLocalNotification: or cancelAllLocalNotifications,
respectively.
So you can cancel the notification and then schedule it again using
scheduleLocalNotification
Here is from the Apple documentation ..
Note: Prior to iOS 4.2, this property was a read-only method. A setter
method has been added and the method has been converted to a
read-write property. When you set this property, UILocalNotification
replaces all existing notifications by calling
cancelLocalNotification: and then calling scheduleLocalNotification:
for each new notification.