I had a similar situation last year, changed a booking by adding a YQ airline. There was no mention of the additional YQ at the end of the call (only the $90 change fee), I was pleasantly surprised at that. I then received a call back about 1 hour later from a supervisor informing of the YQ. He wouldn't/couldn't budge on the YQ, but did a lot of searching to find me an alternate routing that avoids YQ and then even waived the change fee.

Incidentally, I knew to expect YQ when I made the change, and was grudginly willing to pay the "reasonable" amount for the AC YYZ-HKG segment (for a trip to SYD, and no other YQ airlines involved), but this is when I found out that YQ is calculated as if it was YYZ-SYD. I was not willing to pay that inflated amount.

You may have more of a case if they didn't tell you on the call, or immediately after. At the very least, see if they'll refund the change fee.