I'm not exactly sure what UA codeshare flights booking class is equivelant to the AC ones as I've never booked under a UA flight # operated by AC and then using a cert to upgrade but regardless let's pick one flight and see:

AC799 YYZ-LAX: J C Z Y M U B H V Q L A S R K N G P E T
The United Codeshare for the same flight is UA8337 and displays the following booking classes: F Y B M E U H Q V W S T K L G P A

So I assume the J on AC is equivelant to the F on UA and as for the rest.....the Y M U that exist under the AC flight # also exist on the UA codeshare #.