What happens during "previews"? Would it be open to those that are in the park at that time?
Previews are usually a first viewing of the area for those interested. It is not open to all in the park. You have to sign up and there are various options usually. For example, I did the Cars Land preview when it opened in 2012. We got to ride all the rides, taste the various foods, and walk down Buena Vista Street before it reopened.
For the Guardians Preview, my kids ended up riding GOTG 10x that night. We got to try various new foods. We also got to sit in with the Imagineers while they discussed the crossover of the ride.
We even had our own car with our friends.
For the Incrediacoaster, we got to experience the area, in the first hour, my kids and granddaughter rode the Incrediacoaster 5 times. We also got to try out the new foods, like the Cookies, ice cream, hot dogs. It was a lot of fun.
My daughter is working on riding every ride without other people. This was her and my niece on the Incediacoaster.
I do not expect the previews for SW:GE to be cheap. I am estimating $500-750 per person. Cars Land was cheap, I think it was $50 or 75. I think GOTG was $150, and Pixar was $250.