The first change that was announced, well before the bag fee, was assigned seating. The date I remember given for this change was on flights booked after May. So maybe it will only apply to those flights booked after that date. I would hope there would also be a date when flights would start with assigned seats.
Assigned Seating has always been listed as starting in early 2026. So the tickets that will offer assigned seating will go on sale in late 2025.