Miller is still live at 10AM ET, at least according to his web site.
Last Miller show is March 27, according to the Chicagoland Radio and Media site. Brandmeier will start at WGN (the origin point for the show) sometime before then, and presumably will be plugged into the network as of March 30.
...Jonny B's Chicago flagship will be WLS (partially replacing Limbaugh), not WGN.