Also, both airlines priced and sold you a ticket from A to B. Transit points are not ticketed breakpoints so are irrelevant, unless you booked each sector separately.

UA rebooked you from A to B on a different routing because their contract was to fly you from A to B. QR are also looking for A to B. But if your schedule change was not by enough to trigger an involuntary rebooking then you are stuck with the ticket you booked, or rebooking under the fare rules (subject to availability).

Neither airline appear to have entertained changing A to B into A to C, or into C to B. And rightly so, that is not what was purchased.