This is for existing orders. The line item has not been fulfilled. The credit card payment has been authorized but not captured.
We use the line item properties to store certain custom information, and we sometimes need to change that custom information (and sometimes a customer managers to checkout without filling in that information, so we need to add it to the line items that are missing the information).
We are okay with removing a particular line item and then adding it back again with new line item properties. I just can’t seem to figure out how to add a product to an existing order and include line item properties.