Ed,
You have to remove the currently selected item, add your new item, then select it. See lines 52-58 below
0010 print 'CS'; list
0020 list_box 100,@(2,14,12,6),fnt="*"
0030 list_box load 100,"Dog/Cat/Pig/"; wait 1
0040 list_box write 100,"Cat"; wait 1
0050 ! list_box load 100,1,*
0052 list_box read 100,_index ! Get index of current selection
0054 list_box load 100,_index,* ! Remove current selection
0056 list_box load 100,_index,"T-Rex" ! Add this one in its place
0058 list_box write 100,"T-Rex" ! Select the new option
0060 end