Product Exchange with Local Delivery

[thecoveteaco]

I have an order that needs to split their chosen product into the chosen product and another product, but because they’ve selected local delivery, I’m unable to do an exchange of products in the system. 

Please help as I don’t want to burden my customer with a refund then a new purchase which wouldn’t be under the same order in the system. I need to seamlessly exchange a certain quantity of the first product and order the “returned quantity” in another product for their order. 

How do I do this?