You're right on the nose, Elphie3. This was planned from Day One. Two casts were hired. One would play out the first three weeks of previews, then the second cast would come in to finish out previews and open the show.
I met two of the (first) cast members at the stage door of 110 IN THE SHADE, and they were telling Steve Kazee to come see them soon because they were only in it for the first few weeks after which a second cast came in.
"Winning a Tony this year is like winning Best Attendance in third grade: no one will care but the winner and their mom."
-Kad
"I have also met him in person, and I find him to be quite funny actually. Arrogant and often misinformed, but still funny."
-bjh2114 (on Michael Riedel)