You might try telling them we are going to be in X destination for 4 days, and have a schedule of things we want to do. Now, with a couple hours chopped off of it (I am presuming you now have to leave earlier), we are not going to be able to do everything we planned.

However, if we could leave a day earlier, we could still do XYZ that we had hoped to do, by getting there one day earlier.

Would that be possible?

Ask real nice, and have some story (like above) that makes some sense to the reservation agent, and you might have a chance.