Do you know whether your original award ticket priced (in miles) as a RT or two OWs put together on the same ticket/PNR? If you got RT pricing initially, I think it's standard with DL that you must cancel the whole thing and find new award availability. Similarly, it could be difficult to try to just fly the outbound and then cancel the return and expect to have the miles for the return portion refunded to you. I know this issue has been discussed here in the fairly recent past, since DL got rid of its award pricing charts.