When changing anything in the assets\minecraft\blockstates\piston_head.json file of your resource pack, no visual change happens in-game as extended piston heads completely disregard their blockstates file. An example is the attached resource pack, that should change all extended piston heads to appear as sticky pistons and also change all piston heads' rotation to downwards; however, as you can clearly see when loading up the resource pack in-game, it does not.