Moderator, Air Canada; FlyerTalk Evangelist
Join Date: Feb 2015
Location: YYC
Programs: AC SE MM, FB Plat, WS Plat, BA Silver, DL GM, Marriott Plat, Hilton Gold, Accor Silver
Posts: 16,834
That's two separate changes. I'm not 100% sure whether the first airport can make changes. Generally when I'm flying LGA-YYZ-YYC and I take an earlier LGA-YYZ, I have to make the change to the 2nd leg after I land in YYZ.
Of course, on YYZ-YUL you can go standby early for free.