Had a similar experience back in March during the 5 day RDF shutdown of YYT. My flight to YUL got cancelled so I missed my TATL to LHR & essentially the whole carefully crafted itinerary of a 10 day trip to Europe was going to be shot to hell to the extent that cancelling the whole thing was the likely outcome. Through a bit of lateral thinking and research and a couple of attempts through the E75 call centre number, I managed to get a helpful agent to change my outbound from YYT-YUL-LHR to YDF-YHZ-LHR. That was actually the easy part aside from the 8 hr drive to Deer Lake the following day, what was more complex was the onward SAS bookings which they did manage to slightly screw up leading to a tense hour or so in OSL later that week wondering whether I was going to get on the flight back to LHR or not. The agent that made the changes for me was patient & helpful and it was a good experience.