Sitting in the United Club at LAS waiting for AC 1854 red-eye to YYZ when I get a flight update text saying the flight is canceled. Seriously, 90 mins before departure?! So, I call AC and while the agent is looking it up, the UC ladies come over to tell me it's a flight number change only to 1994. AC agent confirms same. So what would cause AC to change a flight number 90 mins out?