It can mean a few things.
1. They have sold out of the cheap fares and won't be offering any more
2. They think they can sell at full price and will wait to see what happens before they offer any discounts
3. They are selling so well at full fare, they have no need to offer discounts.
4. If ALL fare classes are unavialble, the flight is full... changes can happen, but your chances of a discount just dropped dramatically. Cancellations do not retain the fare class originally assigned in most cases.
Considering how popular President's Day week is, if you see a fare you can live MOVE QUICK, don't think about it.