Has anyone been able to skip the departure leg but fly back on the return leg of a round trip ticket?

I bought a round trip flight, but due to personal reasons must depart earlier. I plan to use Aeroplan miles to exchange a One-Way ticket, but hope to return on the previously booked round trip (not boarding the departure and taking a loss, since changing the departure flight is more expensive than converting miles)

I'm told that the entire trip is cancelled if I don't depart with the original round trip itinerary. Is there a way around this?

Thanks!