Need to change item from "Non-Inventory" to "Inventory"

I have a user trying to change the Inventory Item check box (in Define Item page) from "not checked" to "checked", but the box is grayed out.

There is a requisition, PO, and receipt for this item.

According to user, this item has been received and since the invoice has already processed, I’m wanting to manually add to inventory instead of using the receipt process.

Any ideas on this? Can this flag be updated? If not, what are options for resolution?