So you are just speculating.
There would be no reason to open ahead of time with the old film. Not only would guests be even more disappointed but logistically why would they go through all the work just for a few weeks? It's not like you are just switching out a DVD. The gliders have to be programmed and calibrated with the new movie. CM are going to be spending some time "flying" while they test all of it. It would make no sense to go through all that, change it, change it again, go through all that again. And we have the smells to switch out as well.
And if Frozen "land" opens the end of May the last thing they need is more chaos in that park those first few weeks.