This topic comes up every now and then, it all depends on the booking class of the operating airline the booking class of the codesharing carriers translates into. In your case, a UA W would never translate into AC W, since AC uses W for award tickets in economy, wheras UA uses X for that.

The booking classes that the UA agent wrote to you are AC's fare classes that you can not see on your ticket if you book an AC operated flight with a UA flight number.

Check out the Fare codes for UA tix on AC metal? thread in the AC forum for some further information.