Hi,

I presently share a joint AA booking with a friend that flies from YYJ-SFO, with a short three-day trip there, then returns home to VCE, using the flight path
SFO-YVR-LHR-VCE.

I'd like to (myself, not including my friend), make a week-long stop in YYZ on my way home... this would likely look something like SFO-YVR-YYZ-LHR-VCE, or similar.

Is it possible to change the booking for just myself, and preserve the flight for my friend, who wants to return directly home? Which fees would be incurred? I believe the booking was done through an agent- how does this change the process?

Thanks in advance.