United changed my nonstop to a two-legged itinerary. What's my recourse?

This kind of change is terrifically frustrating, and while not especially common, can happen on any airline. United normally sends emails when there is a schedule change, as well as providing notifications via its mobile app and website.

_{Long ago prior to the Continental merger, they would also give me a phone call if I did not acknowledge the change within a day or two. I am not sure if that is still the practice, as the events of 3/3/12 were the last straw for me, and I went to the dAArkside as they say after many years of loyalty to the blue.}

This is probably not what you want to hear, but United has little in the way of legal obligation to you, because they have held up their side of the contract, in which they only promise

• to transport you from your origination city to your destination city
• in the ticketed class of service
• within 7 days of the original scheduled arrival.

There is no superseding law or regulation I am aware of in Hong Kong SAR or the U.S. that would compel them to provide a certain amount of compensation for your inconvenience.

They also promise to attempt to communicate changes to you in advance, and though you did not receive this notice, they can argue that they made reasonable attempts to do so, especially if other passengers did receive the notice.

Even if you did receive advance notice, you cannot generally insist on a particular routing—that is, you could not demand that United rebook you on Cathay Pacific, which is the only airline currently operating HKG-ORD nonstops. You agreed when you purchased the ticket to allow United to modify your itinerary (more on that later), with the only recourse being an involuntary refund.

All that said, on account of the involuntary rerouting you can ask United to reschedule your return flight for more convenient times—though you will almost certainly be routed through SFO regardless. There is more capacity than from EWR, GUM would require a double connection via HNL (and is ending soon), and NRT and LAX service ended some time ago. As noted in the contract, they will not book you on a different airline over this.

A calmly worded complaint about the inconvenience and additional stress this involuntary rerouting has caused you, not to mention physical pain, may lead United to provide voluntary compensation in the form of a voucher or frequent flyer miles, particularly if you are an elite member of Mileage Plus.

What an airline promises you is outlined in its contract of carriage (also known as the conditions of carriage, and like terms), which you agreed to when purchasing the ticket.

The terms for passage on most major airlines are substantially similar, and I would not be out of turn for saying they are designed to protect the airline rather than the passenger. United's contract of carriage covers your scenario under Rule 24 (emphasis added):

A.3. Schedules are Subject To Change Without Notice - Times shown on tickets, timetables, published schedules or elsewhere, and aircraft type and similar details reflected on tickets or UA’s schedule are not guaranteed and form no part of this contract. UA may substitute alternate carriers or aircraft, delay or cancel flights, and alter or omit stopping places or connections shown on the ticket at any time. UA will promptly provide Passengers the best available information regarding known delays, cancellations, misconnections and diversions, but UA is not liable for any misstatements or other errors or omissions in connection with providing such information. No employee, agent or representative of UA can bind UA legally by reason of any statements relating to flight status or other information. Except to the extent provided in this Rule, UA shall not be liable for failing to operate any flight according to schedule, or for any change in flight schedule, with or without notice to the passenger.

[...]

C. Schedule Change- When a Passenger’s Ticketed flight is affected because of a Schedule Change that modifies the original departure and/or arrival time by 30 minutes or more, UA will, at its election, arrange one of the following:

1. Provided that the dates of departure and arrival must be within 7 days of the originally scheduled dates of departure and arrival, respectively, transport the Passenger on its own flights, subject to availability, to the Destination, next Stopover point, or transfer point shown on its portion of the Ticket, without Stopover in the same class of service, at no additional cost to the Passenger;
2. When a Schedule Change results in the cancellation of all UA service between two cities, at UA’s sole discretion, UA may reroute Passengers over the lines of one or more carriers in an equivalent class of service;
3. Advise the Passenger that the value of his or her Ticket may be applied toward future travel on United within one year from the date of issue without a change or reissue fee; or
4. If the Passenger is not transported as provided in C) 1) or 2) above and does not choose to apply the value of his or her Ticket toward future travel as provided in C) 3) above, the Passenger will be eligible for a refund upon request. See Rule 27 A).

Instead of accepting the modified flight, you could have demanded a refund, but the amount of the refund probably wouldn't cover any alternative purchased on the same day, so you probably didn't lose out there. I am not sure if you can still demand a refund for the return portion of your ticket, since you did accept the modified itinerary, but if you want to push for it, the guidelines are listed under Rule 27.