I have a partially used round-trip ticket booked through a travel agent. The remaining segment is from YYZ to HKG in economy. I want to issue biz award ticket on the same YYZ-HKG flight.

I need to cancel the economy ticket before I issue the biz ticket to avoid duplicate bookings.

Here is my question: can I contact the airline to void/cancel the remaining YYZ-HKG flight? Is it always free of charge if I do not need any refund?

Thank you very much for your kind advice.