Had posted this in a separate thread but need a response soon as my flight leaves in <24 hours

I have an ICN-SEA-SFO flight on OZ/UA in J booked with Aeroplan miles. Due to an equipment change, the ICN-SEA leg is now on an A333 with OZ's old J. I would like to rebook/change to another flight (I'm finding availability on OZ ICN-SFO or on EVA, ICN-TPE-SFO).

I understand I'm within the 21 day cancellation window where I would have to pay a fee to change my flight, but would I be able to do that online by cancelling my booking and rebooking one of the other flights I found (I know I'd still have to pay the fee) or is that something that can only be done by an agent over the phone (rebooking a cancelled itinerary)?

Thanks!