Originally Posted by gleff
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.
UA does comp other *A. As I just went through the process, and did call everyday to "check" the status, I've landed on a very nice agent. Once I've told the story she said "but we don't comp from *A". I engaged her in a conversation trying to ask why exactly is that - is not that I'm going to get anything new (i.e., *G will get you exactly the same boarding zones and priorities on UA as 1P), just the miles will be deposited to another account (granted there are differences on how many miles you get in each program but that is not the point). To make a long story short she said, let me check something out, she pulled up the list of programs UA comp from, and low and behold LH was there. Needless to say I've got comped to 1P.