Here is the current "late openings" as listed on the Disney site.
Dumbo the Flying Elephant----May 2----12PM-10PM
The Barnstormer----May 2----12PM-10PM
Under the Sea-Journey of the Little Mermaid----May 2----12PM-10PM
The Many Adventures of Winnie the Pooh----May2----12PM-10PM
New Fantasyland will be "slammed" that afternoon after the Dedication,but later in the day,I don't think it will be any worse than "normal".