What happens if an item with a material cost becomes a structure?

If this occurred in the same revision, the original material cost is removed and becomes read-only for the structure or assembly.

Note that it could still be stored in the schema so that it can be used in a future release. If this occurred in a newer revision, the material cost remains visible for the previous revision of the item, when it was a leaf item. In the new revision, the material cost is rolled up from the child items.