They can't just remove the leg, they sold you a ticket from YVR to OSL. So they will need to put something else there. You can't wait with dealing with the change until next summer, as that would probably been seen as you having accepted the changes given.

Technically, changes should probably only be for the return portion, but it probably depends quite a bit on the agent what you can be allowed to do. If it was me, I'd look at 1 what would be your route you'd want back replacing the missing leg from YVR. 2. If I could change everything, what would I like.
Then call and start from asking for 2, and see how it goes. If no nice on changing the first two legs, then get to the point of getting a proper itinerary back