Since 1.14 projectiles like Snowball, Enderpeal, and Eggs extend the NMS class 'EntityProjectileThrowable'. This class has a method setItem which accepts an ItemStack to change the displayed / thrown item. This can be used to make any ItemStack throwable or used as a projectile.

 Example:

Snowball projectile = player.launchProjectile(Snowball.class);
((CraftSnowball) projectile).getHandle().setItem(CraftItemStack.asNMSCopy(new ItemStack(Material.CARROT)));

It would be great to get this implemented to the API with a setter and getter method.