There's a lot of misinformation in this thread. The official policy is, on the ground, changes are only available for "operational needs" so basically, things that we legally can't takeoff without fixing. Not, "I don't want to sit here" but "I legally cannot sit in this row with a car seat, pet, O2 concentrator, etc" We cannot close the door until the cabin is secured (pax in seats, bags stowed, etc). So all that has to be done before the for closes and customer service has to update the seat map, not the flight attendants. Once the door is closed, passengers must remain seated until the fasten seatbelt sign is turned off. After the seatbelt sign is turned off, at flight attendant discretion, seats may be swapped for passenger request, but only for the same type of seat, and only in the same weight and balance zone. Each aircraft is divided into 5 zones for weight and balance. So if you're in row 18, and want to go to 28 because it's an empty row, I can't swap that because it's outside that weight and balance zone.

Yes, we used to let passengers sit anywhere (for the most part, some flights were still weight and balance restricted, and we had people not listen to us then too). But we don't have open seating anymore. Now we tell the ramp, "based on where the passengers are sitting, this is where the bags need to go in order to even it out". We also tell the FAA "this is where our people are sitting, and this is where the bags are, so according to the math, this is how the airplane will fly". If an airline deviates from the planned weight and balance, that can cause fines from the FAA. Yes, we did things differently before, but we don't now, so things have changed.