I would change that ressie...
If you are planning on being up that early then I certainly wouldn't waste AM EMH in a restaurant. What I would recommend is to get either the latest breakfast or earliest lunch ressie. Then you can tour the park, you will get so much done, and by the time your ADR comes up, you will be so ready for a meal.
We did this. It was an 11:20 AM ressie for lunch. Very good. Get over to the podium early because there can be quite a line just to check in.
If you keep the ressie, you should be able to get into the park before everyone else. At AK, they had a special turnstyle that was designated for the character meal ADRs. They will verify that you are on the list and then admit you to the park. You might want to get there extra early in order to get some pictures without people (if that's your goal) because sometimes at MK they let people into the park and up to the castle area where there are ropes hanging there for what is open.