This article in Sun-Times sums up what’s going on.

http://www.suntimes.com/output/news/cst-nws-park08.html

The impetus for the move is that the Navy Pier location becomes very crowded during summer months.

The museum would be underground (“below grade”) and located next to the Grant Park’s Daley Bicentennial Plaza. It is believed that traffic would flow largely on intermediate Randolph. Residents believe this could lead to some congestion at the intersection of Randolph and Michigan but otherwise the top level streets Upper Randolph, Upper Columbus, and Upper Harbor would be largely unaffected. However, the outdoor playground in Grant Park next to the Daley Bicentennial Plaza would presumably get a lot more use.

Construction would last approximately 18 mos.