I think you'd see the biggest effect at the margins... Not people moving trips from peak season to value season, but rather visiting the parks on the weekdays during the trip with break/waterpark days on the weekends or arriving a day or two earlier to "beat" the seasonal increase. I also think it'll push people away from adding extra park days "just because", as we often do now with the lower per-day pricing on longer tickets, because more park days will frequently mean running into a higher season (based on the survey that was floated showing virtually every weekend as a higher price tier than the surrounding weekdays).
I wonder what it will mean for the long "party season" at the Magic Kingdom. Theoretically, they could probably demand top-tier pricing for all of the full days during those months because they're so desirable (and so crowded) due to the short hours and lack of evening entertainment on party nights. So it could logically come to a point where you have to pay more to see any evening entertainment at all for 1/3 of the year.