Depends on when you are going. Christmas week, Spring Break, Thanksgiving week or other super busy times, park openings can change to 8am or even a little earlier. Any "normal" to low crowd times, 9am openings don't change unless a morning EMH gets added.
To answer your question, though, WDW can change their opening and/or closing times whenever they want- up to the same day. But the likelihood of them changing park opening times for non-crowded times of the year is very low.

Depends on when you are going. Christmas week, Spring Break, Thanksgiving week or other super busy times, park openings can change to 8am or even a little earlier. Any "normal" to low crowd times, 9am openings don't change unless a morning EMH gets added.
To answer your question, though, WDW can change their opening and/or closing times whenever they want- up to the same day. But the likelihood of them changing park opening times for non-crowded times of the year is very low.
Excuse my ignorance, but why would it matter?I have an 8:05am BOG ADR on Tuesday August 18 and the MK opens at 9:00am.
I'm hoping since we're less than 3 weeks out the park opening time won't change to 8:00am!
I think they don't want to be eating breakfast while the park is open. They would rather be able to eat and get done by the time all the park guests are entering.Excuse my ignorance, but why would it matter?
Oh! Sorry for the misinformation- I stand corrected. We have never personally experienced changing park opening times, but we tend to visit during the least crowded weeks of the year.This is not correct. The MK in particular adds non-EMH early openings quite often during moderately crowded times.
If park opening time changes the night before, how are guests supposed to find out? Just curious - our first trip is in early September. Thanks!
Is it typically only the MK that has time changes or does it happen for all the parks?
Excuse my ignorance, but why would it matter?
I think they don't want to be eating breakfast while the park is open. They would rather be able to eat and get done by the time all the park guests are entering.
