Can I swap a maneuver known with another one if I have te requisite before the swap but stop having it after?

Question

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?

Answer 1

The rules don’t really cover this. The only thing we have to go off of is the text in the crusader/swordsage/warblade descriptions themselves:

Upon reaching 4th level, and every even-numbered swordsage 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.

^{(Tome of Battle, page 15—note near-identical wording on pages 9 and 21 for crusader and warblade, respectively)}

Prerequisite

Manuvers Known: Some of the more powerful maneuvers require you to learn one or more other maneuvers in the same discipline before they can be selected.

^{(Tome of Battle page 44)}

This is it, literally everything written on the subject of swapping maneuvers and meeting their requirements. We have nothing more to go on. The phrases “in place of,” “in exchange for” seem to suggest that the old maneuver and the new maneuver don’t ever co-exist, but that’s unclear and in any event, the requirement must be met in order to “select” the new maneuver, and the swap doesn’t really take place until you have chosen both the new maneuver (so it’s already selected), and the one to replace (which goes away but it doesn’t matter).

(One thing we can be sure of: losing the prerequisites for a maneuver doesn’t make any difference after you’ve selected it. If a 9th-level swordsage learns soaring throw, and then at 10th swaps out mighty throw and at 12th drops devastating throw, leaving them with soaring throw as their only Setting Sun maneuver, they can still use soaring throw even though it requires “Two Setting Sun maneuvers.” That only applies for the selecting.)

Anyway, realistically, you should just ask your DM. I have been at plenty of tables that forgot maneuver prerequisites were even a thing, and plenty of others that remembered them just long enough to houserule them out, so I, for one, would be happy to let you make this swap. The maneuver requirements just don’t add all that much to the game.