The old terminal will stay. The expansions will be placed to the side and behind.
lbreport.com has been tracking LGB with lots of discussion of the expansion and other issues at http://www.lbreport.com/airport/airport.htm
They've posted the proposed improvement drawings on their site.
Map of the terminal layout with the additions
LGB also updated their traffic reports through Oct 2002. Year to date passenger figures were up 125% versus the first 10 months of 2001.