Orders to change equipment took some time...
I know this is the British army but to supply the troops in Canada while dealing with armies in the Peninsula too would have been quite straining. Already in the Peninsula soldiers had different colors of pants and that was Britains primary front.
Most British infantry had already replaced the breeches with white trousers when serving abroad by 1808. The orders to officially replace the breeches with blue-grey trousers would have reached Canada by 1811, so by the start of the war in July, 1812, any remained breeches would have been worn out and replaced---when you only have one pair of leggings they don't last very long at all, certainly not a year. That's why British troops serving more than a year in the Peninsula almost invariably wound up in brown, locally-made trousers (or even coats, in some cases).
I can't see your video presently as all Google pages, including YouTube, are currently down. When you start using reenactors as reference you risk making huge mistakes, though, as some are accurate, whereas others just don't really care. I'd say the majority fall into the latter category.