Set a location at where you want this change to occur then use a "Bring" condition trigger and a "Remove Unit at Location" and "Create Unit at Location" action triggers.
by point in the game do u mean location, or time into the game, or kills?
if it's time into the game or kills you would have to create a location around the unit and have your next unit spawn at that location and then kill off the previous unit
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.