What’s the deal with flight numbers changing all the time?

Over the last several months, I’ve had a fair number of “schedule changes” that required attention. In almost every case, the only change was the flight number. Sometimes from one four digit number to another four digit, sometimes a three digit to a four digit, and vice versa sometimes. I think I’ve had more flight number changes in 2025 than in my entire flying life with United combined.

What’s the deal?

I can understand that if/when UA starts a new route, it needs a new number, and if for whatever reason, it wants to use a number in use elsewhere, it needs to swap out. But from what I can tell, none of my flights with changed numbers had any special numbers to speak of. Does anybody know what’s going on here?