Originally Posted by sinoflyer
When a flight (usually international) uses the same flight number but continues onward using a different aircraft, as is the case for UA936 (ORDIAD is a B777; whereas IADZRH is a B763), UA has a practice of using the nose number for the call sign on the domestic segment. This way, in case of a delay, they can dispatch the second segment without causing confusion to air traffic control. I guess they learn from experience.
This is not correct. They do not use the ship number, they have a fixed alias flight number. An example is UAL8146 referred to by the OP; it runs from ORD to IAD according to flightaware then continues to ZRH with a different aircraft that uses the flight 936 call sign.
http://flightaware.com/live/flight/UAL8146
Why they pick the numbers they do is a mystery; it seems to me that it would make more sense to use the prefix "8" followed by the marketed flight number. The 9900 and 9800 series of flight numbers are used for ferry and charter flights, but the 8800 and 8900 series are unused. In this case, the domestic segment of 936 would be 8936 instead of 8146. The other aspect of this is that they are not consistent in the use of these alias flight numbers; 888 is used on both the PEK-SFO segment and the SFO-LAX segment.