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.