Well, Soarin or Everest had longer soft openings because the attractions were ready long before the official openings.
As for Buzz Lightyear in DL Park last year or Toon Studios, there will be barely enough time to have soft openings. So I wouldn't bet on a May one. I know the attraction will be finished by mid-May and given back to operations, but it is still unclear if they will held unofficial previews. Maybe if you have luck.....