Change a bi-directional 'Dropdown' lookup field to 'Multiple Select'

Change a bi-directional 'Dropdown' lookup field to 'Multiple Select'

I have an existing lookup field that relates Sales Invoices and Shipments. When I created the bi-directional relationship, it was a 'Dropdown' field. I would now like to enable multiple Sales Invoices to be tied to a single shipment. Can I change the lookup field to 'Multiple Select' in the Shipments Form i.e. to allow more than 1 Sales Invoice per Shipment (I don't need more than 1 Shipment per Sales Invoice)?