It is quite normal for them to adapt the hours closer to the time, however the busier months are more likely to be affected. They generally only add hours/shows/parades but it is highly unlikely that they will reduce them or change the actual EMH days. It can happen but it is not something I would even worry about. Keeping a park open later shouldn't impact your plans in any way, unless you were hoping for an early night and can't resist staying up later if the park is open ...
The only thing that may affect touring plans, if you follow one, are extra parades or particularly a show like Fantasmic, which will have an effect on crowd levels.
Just keep an eye on the hours and on the boards so that if it does happen you can take the changes into account as soon as possible. The general trend is for Disney to make these changes quite close to the time, as in a few weeks (or even less on occasion). Again, unless you are there over the Thanksgiving week/weekend I don't think you have much to worry about.