Flight numbers get changed when airlines re-do their scheduling. It's still a fairly manual process as sometimes the numbers are symbolic.
Either way, when they talk about re-using flight numbers on the same day, what they mean is they create multi-stop flights to use the same number for each segment instead.
So you'll see flights like, hypothetically...
> AA 1325 ORD-LGA
> AA 1351 LGA-YUL
> AA 1352 YUL-LGA
> AA 1326 LGA-YUL
Getting converted into a single flight number.
> AA 1325 ORD-LGA-YUL-LGA-ORD
Basically converting this to a flight from Chicago to Chicago via New York and Montreal nets them a 4:1 reduction in numbers used.
You can still book each segment separately, and if they generally book the same plane, say some Envoy Air regional jet, for all four segments, there's little risk that irregular operations will lead to two planes in the air at the same time with the same number. Worst case they can give that one segment a new flight number for that day.
They've been doing this a lot in the last few years, especially on regional flights, and especially in the northeast where there's a ton of short hops.
[edit] If they want a bunch of their flight numbers back they can stop codesharing and switch back to relying on interline ticketing and sell the operating carriers flight number. I can't really think of anything good that's come of codesharing to individual passengers.
Either way, when they talk about re-using flight numbers on the same day, what they mean is they create multi-stop flights to use the same number for each segment instead.
So you'll see flights like, hypothetically...
> AA 1325 ORD-LGA
> AA 1351 LGA-YUL
> AA 1352 YUL-LGA
> AA 1326 LGA-YUL
Getting converted into a single flight number.
> AA 1325 ORD-LGA-YUL-LGA-ORD
Basically converting this to a flight from Chicago to Chicago via New York and Montreal nets them a 4:1 reduction in numbers used.
You can still book each segment separately, and if they generally book the same plane, say some Envoy Air regional jet, for all four segments, there's little risk that irregular operations will lead to two planes in the air at the same time with the same number. Worst case they can give that one segment a new flight number for that day.
They've been doing this a lot in the last few years, especially on regional flights, and especially in the northeast where there's a ton of short hops.
[edit] If they want a bunch of their flight numbers back they can stop codesharing and switch back to relying on interline ticketing and sell the operating carriers flight number. I can't really think of anything good that's come of codesharing to individual passengers.