I booked YVR-FRA-MAD in P class, with FRA-MAD on LH (AC code share). 48 hours after booking I realized that FRA-MAD was in Y class.

The fare difference to P is $1 and reservations quoted me the same - but with a $500 change fee. They wouldn't budge on the fee.

My mistake for booking the code share in the first place, but this seems like a reasonable request that is not technically a "change" but an "upfare".