<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by InTheAirGuy:
I'm off to the US today -- a full fare J ticket that is about $1,600 all in. Got here early -- there was a UA flight going an hour early, codeshared with AC.</font>
Full fare tickets should be accepted by any carrier, regardless of codeshares. Next time, just go to UA personnel and ask. They should accept you.
AC personnel probably just didn't want to send you on UA because it would have meant that UA would have got the ticket value for your flight.