If the flight change is for arrival on the same day, it really doesn't matter and you truly don't need to change anything. DME doesn't really care when you actually arrive. They are not going to be chasing you down, holding the bus until you get there or cancel your reservation. The arrival info is only used be DME as a guideline to help them get a rough idea of the passenger loads throughout the day.
The important flight info is the return trip as that determines your pick up time.
Steve