I've had to do this before - as long as they flight hasn't left yet, I always just call and pretend I'm the pax..

" I don't have my PNR handy and I'm enroute to the airport and stuck in traffic, can you confirm what flight I'm supposed to be on? My assistant booked it and I might have to change it to a later flight.."

have never had air canada refuse to grant an opportunity maybe earn a change fee.