The UA fare class should convert automatically to the equivilent AC fare class. As noted this is likely to be L or Q. I recall once booking a LH codeshare with the original flight number being an AC one, but the flight being on LH metal. The AC res had the fare class as L, but the LH boarding pass showed it as W. (LH's passes generally show the actual fare class in the box on the smaller section after the flight number.) I, of course, panicked, knowing on AC a W fare would be considered consolidator/AIR MILES and thus not post for credit. This proved to be the case, but in the end I faxed my original ticket/print out with the AC fare code, the boarding pass, along with an explanitory note, and did get proper credit.

So hold onto your paper, just in case.