Both these phenomenons are not strange these days, nor new.

Express has been using the same numbers round trip for years now, definitely well back into the PMUA days. It's always, IME, when its that same aircraft going hub-spoke, then back to the same hub the same day.

The changing flight number thing is newer, though I seem to remember this becoming more common around merger time. Some speculate this is done as changing the flight number means the on time statistics can be reset and in the early days of it, not reported at all as it needs to be running the same route for the OTP numbers to be required to be shown.

But even PMUA used to change flight numbers somewhat, though they were more consistent especially earlier. For example, way back, probably until about 2007 or so, ORD-YYZ flights were the even numbers of 1100-1115 consistently, with the returns being the odd numbers in that range. But the connecting flights to and from ORD would change. So one day, flight 1111 might be, say, OMA-ORD-YYZ on the same flight number, and after a while, the flight number might be a DFW-ORD-YYZ. Eventually, the YYZ flights got more seemingly random numbers as well, and with the change from all mainline to a mix of mainline and express, it had to.

Also, some flights switch between PMUA and PMCO metal on certain days, so need different flight numbers (yes, still). For example, a couple of years ago I did IAH-YVR which was on UA 320, but was looking at going the next day also, which was a CO 737. So that's a potential other reason.

Never expect the same flight number to run the same route/time more than a few days In a row is what I expect these days, at least for the domestic/Canada/Caribbean network.