I would be surprised if previews would not be in full force in late May/June. Back in 2012, we visiting MK around Columbus Day. Fantasyland was scheduled to open in December. Very quietly, they opened Fantasyland to the general public for the day. The days before it wasn't open. We did Tales with Belle and Under the Sea. I was thrilled. Oh, and because it was so quietly opened, it wasn't busy at all. Everyone had planned for a December opening, so no queaus.
Look for suits and CMs milling around the entrance. This will happen. They won't just open it cold on June 30th. They need time to plan and train.