The Tome of Battle states the following:
Upon reaching 4th level, and at every even-numbered swordsage level after that (6th, 8th, 10th, and so on), you can choose to learn a new maneuver in place of one you already know. In effect, you lose the old maneuver in exchange for the new one. You can choose a new maneuver of any level you like, as long as you observe your restriction on the highest- level maneuvers you know; you need not replace the old maneuver with a maneuver of the same level. For example, upon reaching 10th level, you could trade in a single 1st-, 2nd-, 3rd- or 4th-level maneuver for a maneuver of 5th level or lower, as long as you meet the prerequisite of the new maneuver. You can swap only a single maneuver at any given level.
So, let's say my PC (Monk 4, Swordsage 4) has Mighty Throw as his only Setting Sun maneuver and wants to swap it at level 4 with Devastating Throw. He accomplishes the requisite of having 1 Setting Sun maneuver before the swap, however, he no longer accomplishes the requisite after the swap (unless you count the Devastating Throw itself). Is this swap possible?