you cant change the container in minecarts once youve put one in

It would be really useful if we could convert a Minecart with Chest into a Minecart with Hopper (and maybe the other way too). Right now you have to craft a whole new one, which wastes iron and chests/hoppers. A simple recipe to swap them, or making them drop the minecart + chest/hopper when broken, would save resources and make rail systems more flexible.