There really isn't any need to program an animation change when PSA already allows you to do so without any programming. I did some investigation and found some landing animation names.
LandingLight
LandingHeavy
LandingFallSpecial
LandingAirN: landing from N-air
LandingAirF: landing from F-air
LandingAirB: landing from B-air
LandingAirHi: landing from U-air
LandingAirLw: landing from D-air
etc.
Anyway, when you search through your subactions, you should, by default, some of these animation names for the character you're trying to edit, in the case that you're using an unedited .pac file. However, the names might be different for certain characters, but any animation name that starts with "Landing", is a landing animation. By default, the animations should be placed where they would normally go, so you can assume that the proper coding for the subaction where the animation is would be there (ex. Finding LandAirN would have the coding for the animation, by default, at that subaction that the animation is on). So all you really have to do is find where the animation that you want to change is, delete the animation, and type in the new animation you want it to be.
For example, if I want to change Bowser's landing animation from his N-Air to his up taunt, I would find where his "LandingAirN" animation is (which is on Subaction 67), delete the name, and type in "AppealHi".
Also, you might have to adjust the timers so that the animation isn't interrupted, and copy over the GFX and SFX coding over to the replaced animation. So yeah, coding should be used sparingly, so you don't exceed the max file size limit for the character's .pac file.