Hi guys, newbie here desperate for help. I'm trying to reschedule my February 15 return flight to May 30 and Air Canada wants me to pay $4000+ in fare difference.

The flight is from YOW to PEK, Business Class

I checked on Google Flights today and ticket for May is actually cheaper than February. (Feb 15 costs $4407 and May 30 costs $4205)

Does anyone know how did they calculate this outrageous $4000+ in fare difference? It's almost the same as buying a new ticket.

Originally I paid $4,860 for a round trip ticket, and used the PEK to YOW portion in September.
For PEK to YOW business class, Air Canada seems to charge similar price between round trip and one way.