The earlier you book your travel the more likely schedule changes will occur. You should always watch your e-mail as this is the way you'll be notified of these changes. Often the change not only to the flight number or departure/arrival times but to the type of aircraft. In this case seat assignments are often changed or dropped all together. That is why it's important to check your e-mail and check your reservation if you are notified of a schedule change.
Furthermore, seat assignments are never guaranteed. It is a request which in most cases is honored but sometimes seat assignments can be changed even at the airport due to equipment substitutions, weight and balance issues, etc. When you purchase a seat on a flight you are just purchasing a seat on that flight, not a particular seat. Not using an airline because of a seat assignment change is a bit unreasonable but if you feel strongly about the issue go ahead. This happens on all other airlines except Southwest which doesn't assign seats.
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