As a MP member you cannot use UA miles, or any other UA currency, to upgrade on AC. AC do not participate in the *A upgrade scheme.

Your only hope would be an op-up on an overbooked flight, which even at peak travel times would be rare - obviously AC would process its own elites first. It has happened to me on occasion between YYZ/LHR, but only when they are overbooked.

The only other alternative is to find an AC frequent flyer on the same flight with upgrade coupons to spare. Probably best to "look forward" to the AC Y experience which actually isn't so bad.