AC may or may not agree to a change (this is a grey area), but I think you yourself have hinted at the right approach to take... "given that it is the winter season I'm concerned that with the schedule change YHZ-YYZ is now a very tight connection and I don't want to miss my ongoing flight to LHR. Could you book me on the YYT-YYZ flight number *** to reduce the chances of that happening".

Just don't be surprised if an agent offers to book you on an earlier YYT-YHZ-YYZ connection - then you'll have to explain how an earlier YYT departure doesn't work for you.