We usually fly Northwest (only two carriers at our airport) and as much as I complain about their stiff prices, I cannot remember them ever chanign seat assignments on us.
They changed ours when we flew to WDW in March. For our flight down, we originally had seat assignments in like rows 12 and 15 (2 seats together in each row). When we checked about a week before our flight, we found we were moved to rows 26 and 39 (but still 2 seats together in each row). Since we have a child who can't walk and won't use an aisle chair (a small, narrow wheelchair that can go down the aisle), we had dealt with the Special Needs Department for our seating assignments originally. DH called the airline and got 2 of the seats changed back to row 12, so at least we could get on the plane. We rechecked the seat assigments 24 hours before our flight and everything was OK.
Because we didn't know how long it would take to clear security with the wheelchair with the new security, we got to the airport almost 2 and 1/2 hours before our flight. At the e-ticket kiosk, we got a message to check in at the desk - we figured it might be because of the wheelchair (even though we had used the e-ticket kiosk before with the wheelchair). The agent at the desk told us that "the seat assignments are being made at the gate. Check with the gate agent."
The gate agent said we had no seats assigned in the computer and "since you're flying standby, we'll have to wait and see if anyone volunteers to give up their seats. If you don't get on this flight, we should be able to get you on the next flight." DH explained we had bought our tickets in October and had seats assigned when we bought them, and he showed her our printed airline itinerary. She said that didn't matter, we were not in the computer now, so we were "standby."
Luckily, we had the airline 800 number and a cell phone. DH explained the situation to the 800 number person. When she checked the computer, we were listed there in rows 12 and 39. The 800 number person asked to speak to the gate agent (who did not want to talk - she said "I don't care what they say about their computer, you are flying standby"). The 2 agents finally did talk to each other and we ended up in rows 12 and 39.
On our return trip, we got the seats that were originally assigned to us. I have been told that seat assignments are not "firm" until a few weeks before the flight (people with disabilities can get firm seat assignments 90 days before). Our seats should not have been moved because of the disability that was noted on our reservation, but they were. You can also be moved if someone needs the bulkhead or certain aisle seats with armrests that can be lifted up for easier access. These are supposed to be reserved at least 24 hours ahead, but we have seen people asking for them at the time they check in. Anyone who previously was assigned those seats would end up re-assigned.