its about saving money and time basically the change was implemented when the tour went out. the following is my theory as to why the shiz costume changed;
the tour stops don't always have a convenient spot stage left for a quick change. The old costume involved a full change of the dress and wig, The new one involves as far as i can gather pulling the tiara out of the wig, taking off the gown (the new poofy white skirt is most likely under the gown) and the only real "change" is to bup on the beret and white top.
"Grease," the fourth revival of the season, is the worst show in the history of theater and represents an unparalleled assault on Western civilization and its values. - Michael Reidel