i definitely had options on the website for an in-advance change (I don’t think I tried on the app) - second segment of connection was canceled (route re-start delayed). Was put in on a double-connection instead of better alternatives that existed slightly outside of the allowed parameters. My original routing was SJC-DEN-YYZ. They re-booked me SJC-DEN-ORD-YYZ, which was not optimal.

website followed the given parameters for in-
-advance schedule change, which is UA/UAX metal only, same depart/arrival airports and one other thing (maybe within 2 days of original departure?). Better options would have been to change the DEN-YYZ segment to the AC-operated nonstop, or go from SFO, which has multiple 1-stop options. I got no pushback when I called in and asked for SFO-ORD-YYZ, on a much more expensive option (I was booked in G or K, I think, and re-booked into H, which was lowest available at the time on those flights that worked out time-wise). So that worked out well. If I wanted to do it, I bet they would have put me on an AC nonstop SFO-YYZ as well.