Amalia-
I love going to previews and I don't think you should worry about seeing any of those shows in previews. After Eight is right: usually changes that are made insignificant; a line is added here, a joke that falls flat is removed there.
Even if you got to see something like 13 when it was two acts instead of an intermissionless 100 minutes that might be more fun to experience. You could say you got to see something few did. (Well I guess if anyone saw ANY version of 13 they could say they saw something few saw, ha.)
Kinky Boots and Matilda are too tech heavy for the creative team to make many substantial changes. The worst thing that might happen at either show could be a technical mishap (like what happened at Billy Elliot all the time during early previews.)
If you want to see Breakfast at Tiffany's don't wait until a later trip as it may not be playing 6 months from now.
Marie: Don't be in such a hurry about that pretty little chippy in Frisco.
Tony: Eh, she's a no chip!