Agree with the others that the earlier flight gives both you (and UA) more options. There are many permutations of what could go wrong, but bad news does not get better with age and as time passes, you and UA run out of options. For example, UA could conceivably rob the plane from the later departure to buy time if something were to happen with the plane scheduled for the earlier one. None of that is in your control of course, and won't be known until the day of...and I guess that is kind of the point. If you don't need to be in London punctually, you could roll the dice and stay with your current flight. If you need to be there, or just don't want to deal with the worry/stress, it may be worth considering switching to another option. For me personally, I'd rather be in London figuring out something to do in the morning (and watching any EWR issues unfold from a distance) than scrambling trying to rebook for 24 hours later during a busy summer travel period.