In general. The first thing you will be impacted by is the fare bucket availability. If you have a P fare and want to change flights - if you don't find P - then you will automatically go up a fare. (P to Z adds around 1200 P to D adds around 2400 . . and then C is 3200 again more on top of D and then J is 3200 more again on top of C - so from P to J - you can add 8,000 - - That's the main driver).

Second - There currently is a discount period from Feb 15-19 - so that week is already on sale - and outside of that week - the fares are going up around 700.

Third - You might have a change fee - P Fares YOW-PEK have generally 200 CAD change fee.

So - There is your main reasons. - If they are asking 4000 more - you likely chose flights which had possibly a C-Class availability - which is quite full - So I would look for a P-Fare flight and then aim to rebook on that - and you are looking at ~$1,000.

Hope this helps.