First define your first panel:

Code:
PANEL inventory_panel{
	bmap img_inventory_panel;
	layer = 41;
	pos_x = 315;
	pos_y = 202;
	flags = refresh, transparent, overlay;	
}


then second bmap:
bmap Img_inventory_panel2 = "panel2.tga";

then
Code:
if(something){
     inventory_panel.bmap = Img_inventory_panel2;
}