Comps within Star Alliance are rare. They're doubly rare when it's from one carrier to another based on the same continent (occasionally folks have had luck explaining that they've moved to the other side of the world, so the switch makes sense and isn't poaching from an alliance partner).

Since United is a relatively easy comp you might be able to go from Air Canada to another carrier to United. Downside is that you won't have flight activity with that other carrier to show United... just a printout that shows your status and (if you wait long enough) an elite card. But there's a better shot of this working than of showing them your AC status.