Chicago and Steel Pier did share a producer (PACE Theatrical Group, who seem to have been a pretty big producing force in the 90s, with Tommy, the revival of Grease, and the Dennehy revival of Death of a Salesman amongst their credits). I would hazard a guess that whether they went into this revival with the Weisslers already knowing that Steel Pier was coming in or whether that happened after the fact the discussions to move Chicago over to the Shubert were very amicable.
(Side note: even though Big flopped in there pretty quickly, I forgot just how long of a run Crazy for You really had there)