I'd suck it up and take the earlier flight. The change fee is unavoidable and given how high loads have been all summer, I'd not count on a longer flight like YYZ-IAH to have empty seats if you "miss" your scheduled flight and want to hop on the later flight. Plus, with weather being as severe as it has been, especially in/around Houston, and it being so early in the storm season, I'd want to get into that airport as early in the day as I could, before the persistent daily delays start stacking up. I didn't check AC website to see what the change fee is, but most airlines charge $100-150 plus the fare difference, which I don't consider to be in the "reasonable" category.