From what I understand, it's a complex set with numerous changes of settings. I'm sure that they want to make sure that everything works out as well as possible for a paying audience. They probably don't want any Billy Elliot malfunctions to happen and are trying to smooth out any remaining kinks. I trust Bartlett Sher. Yes, this is the first time he's seeing an original musical all the way through the creative process, but I still have faith. If you see any clips of Light in the Piazza at the Intiman, when Lucas directed it, and compare it to what Sher created at the Beaumont, you'll see just how much Sher's creative input really benefitted the piece.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.