My sister left this am on TRS 733 ROC-LAS…however…when trying to track it, I finally had to use the ‘unknown flight #’ tool. to see the flight number had changed to 733A…to only ‘disappear’ in mid air…to replaced by its ‘proper’ flight #…startling to say the least! Why does this happen?
thanks…forgot to post the links… 
My CO268 coming up on thursday had a four digit number up until 1 November
Possibly to avoid confusion with another flight 733 in the area.
I’ve changed flight numbers mid-flight if there was a similar sounding call sign in the area.
I’ve even changed when there were 2 a/c on the freq with the same call sign.
When I flew for Airnet I used the call sign “Starcheck 13” when I had a pop-up trip. There were times when others were using the same number and it wasn’t picked up by ATC till I was in the air. That can happen when the local controller clears you and you don’t use the “system” to do it.
Probably the most famous case of in flight call sign change happened in 1974 when Air Force 1 switched to SAM 27000 over Missouri when President Ford was sworn in and the onboard Nixon became an Ex-president.
Heard the same thing out here with an Alaska flight. Flt# 91 was changed to 91A because another Alaska flight was using the same Flt #.
Interesting fact about AF1. I’m wondering if the same thing happened when LBJ was sworn in. Or was he sworn in on the ground?
LBJ refused to takeoff until he was sworn in. They were on board AF1 but still on the ramp.
Wikipedia. He was sworn in with Jackie by his side, with blood still on her.
I can see if the same airline uses the same number at the same time, but what about 2 airlines using the same number going the same route? Delta and American both have #127 JFK-SAN, and sometimes due to delays, they will arrive one right after another. Which means they would likely depart and go 5+ hours like that on the same route. But I’ve never heard or seen either of them with the extra letter on the callsign.
flightaware.com/live/findflight/KJFK/KSAN
Plus, JFK’s body was on the flight…
Flight numbers are changed to prevent two flights with the same call sign. For instance: Continental flight 231 is coming in from SAN to IAH. It is two hours late. I am flying CO231 from IAH to FLL. I am flying a different aircraft and I am leaving on time. They change my callsign to CO2310. When CO231 lands in IAH I am halfway across the gulf. ATC then tells me my callsign is now back to CO231. This helps ATC since they handle CO231 every day and know it always goes to FLL. They don’t have to keep looking at my strip to see where I am going.