sburke is right, Triggers are tied to AI orders (either tied to an AI order your side initiates, or to a "painted" terrain area that the enemy moves into), and therefore involve movement by one side or the other. I suppose a person could paint the Trigger Order zone in the same location as the unit's set up zone--so the AT guns don't actually move--and have the order for that trigger be a change of the Ambush range. Why someone would want do that though, is hard imagine.
EDIT: Never mind, Pete's the expert, I'm a slow typer, and pics are worth 1000 words.