How to remodel armor (not the item)?

I want to change the elytra model, but I can't figure it out. I only managed to change the appearance of the elytra in the GUI, but I want to change the look of the worn armour/elytra in third person mode. How can this be done?

The outside appearance of worn armour currently cannot be remodelled. This includes elytra, pumpkins, heads, end rods, banners and everything else you could possibly wear.

You can only change the textures, those are (confusingly) located at assets/minecraft/textures/models/armor and assets/minecraft/textures/entity/elytra.png.
Everything else that can be worn just uses the block texture and model.

Actually the fun part is, Pumpkins can be remodelled and it does change the look of it on their head

Also, you can change the armour model In 1.12... for unknown reasons it was removed in 1.13 and is now impossible. But if you just rewind you game version, you can edit the armour freely!