How to make RMA of Substitute Items?

Dear All,

We have a requirement that One Sales Order having Item ABC is delivered to customer and Sales Order is Closed. Invoice is also completed. Now after some time customer sends item back to our warehouses, now we need to make RMA of it. But here item ABC is inactive and for that new item code DEF is created. Now how can I make RMA with new Item code ??

I need to have the reference of Invoice in Return Line of Sales Order. As I am entering new item code, but not able to enter previous invoice reference.

Please suggest that how to resolve this issue?