I understand that, but what I meant was changing from original flight which started at 6PM but is now at 5PM to a different flight at 9PM. Two months later the 5PM flight now becomes a 5:50PM flight, but now I'm in the 9PM flight and that flight hasn't had any schedule change.

I've done this in the past, but the problem is that unless you are constantly watching at this point since you are now in a new flight you're not notified of the schedule change. With the amount of flights on my schedule it's difficult to continue to keep track on flights I'm no longer on. What happens is that I normally change my schedule to find ways to use the extra time (add extra meeting that's perhaps not needed) and resign to just leave later in the day and only notice the change I describe above when doing a SDC and see the changes to the original flight.

What I do now is that when I get a schedule change alert is I just wait it out since I can always change to the late flight for free, and those flights do not fill up as quickly. If it seems like the schedule is going to stick to the earlier departure about a month out I call and make the change to the late flight.