I know that you can change an itemstacks texture on the fly, but is it possible to change armors texture on the fly, without modifying the armor's file

I'm not sure about the on the fly part, but you can make an armor invisible, just change the texture file to all alpha.

I know it's a little late but here's how I do it in 1.4.7

In your Item class

@Override
public String getArmorTextureFile(ItemStack itemstack) {
if (itemstack.itemID == MinerHelmet.itemHelmet.itemID) {
	// change textures to match the state of the light (on|off)
	if (MinerHelmet.getInstance().getLightSwitchedOff()) {
		return "/textures/helmetLightOff.png";
	} else {
		return "/textures/helmetLightOn.png";
	}
}
return "/textures/helmetLightOff.png";
}

Thanks, now maybe I can push my next release in about week

Quick question, how did you get the Boolean getlightswitchedoff to be ItemStack specific?

I would suppose its a keybinding that toggles it and registers the variable into @Mod file or its part of the item class, though first is easier.