We've been at Disney several times when they've altered the times, and added an hour, but mostly we've experienced it at an earlier hour at MK in the morning. It was during typically busier times, like Memorial Day, Fall break, Thanksgiving, so they might be expecting more people.
We try to make a habit of checking hours often before and during our trip. One trip, they changed while we were there. We've found, if they add the hour in the morning, and you rope drop, can often get more stuff done.
We've had where they added an hour at AK at the end, and we were glad, it should mean the safari and animal trails will be open a little longer, since they often close a bit earlier than park closing.
Now, we are more into riding things closer to park close, so if we saw that they'd added later hours then, we'd likely park hop over there in the evening.