I'd suggest using the state method.
http://game-editor.com/State_MethodThat way you can easily write code that can target individual actions the player does (like jumping). But, if you know the animindex values of jumping, you can use it in the same way. Essentially, an if statement checking if the animindex corresponds to a jump animation, and checking if yvelocity>0 (falling) change the animation to your fall animation. That way the if will make sure you only change the animation when the player is jumping, and begins to fall.