The reason you couldn't change the low density structure is because it is defined using the normal/expensive definition:

  {
    type = "recipe",
    name = "low-density-structure",
    category = "crafting",
    normal =
    {
      energy_required = 20,
      enabled = false,
      ingredients =
      {
        {"steel-plate", 2},
        {"copper-plate", 20},
        {"plastic-bar", 5}
      },
      result= "low-density-structure"
    },
    expensive =
    {
      energy_required = 20,
      enabled = false,
      ingredients =
      {
        {"steel-plate", 2},
        {"copper-plate", 20},
        {"plastic-bar", 30}
      },
      result= "low-density-structure"
    }
  },

So you have to change it in .normal not in the root of the recipe.

Thank you for the information!

In this case, I'll leave it at that now, as it works as well as I thought it would.

But for the - maybe - next mod, I know what to look out for!