How to change a model during an action

I don't know a way to change models, but I know how to change model animations depending on the situation, although it requires messing with the code a little.

I managed to do this to make my raptor creature open its mouth and rear back whenever it is not on ground.

If you care to read through, take a look at http://jabelarminecraft.blogspot.com/p/minecraft-forge-1721710-creating.html . Otherwise, here's the short version:

See the part of the code with the void setRotationAngles, where you manage the creature's animations

public void setRotationAngles(Entity e, float limbSwing, float limbSwingAmount, float ageInTicks, float netHeadYaw, float headPitch) {
			this.body.rotateAngleX = 0F;
			
			//head
			this.head.rotateAngleY = netHeadYaw / (180F / (float) Math.PI);
			this.head.rotateAngleX = (headPitch / (180F / (float) Math.PI)) + (float) Math.PI / 4F ;

			//legs
            this.legl.rotateAngleX = MathHelper.cos(limbSwing * 1F) * 1F * limbSwingAmount;
            this.legr.rotateAngleX = MathHelper.cos(limbSwing * 1F + (float) Math.PI) * 1F * limbSwingAmount;
			
			if (!e.onGround && !e.isInWater()) {
				this.body.rotateAngleX = - (float) (Math.PI / 6);
				this.head.rotateAngleX = (float) (Math.PI / 2);
				this.jaw.rotateAngleX = (float) (Math.PI / 4);

				
				
			} else {
				this.jaw.rotateAngleX = (float) (Math.PI / 18);
			}
		}

With "e" standing for my entity, I can make checks like 

if (e.onGround) {
    // do ground animations
} else {
    // do flying animations
}

There are other parameters you can check like inLava, inWater, dimension, getEquipment, getFireTimer etc you can mess with!

thank you for explaining this