Thx for the comments. Update:

I called and did upgrade for flight home only as there were not any mileage upgrades for outbound flight. Premium seats I had booked on way home have been refunded. Used 15k miles x 2.

It was a bit of an ordeal as the first agent I talked to did not know that the S fare I booked outbound was a special refundable fare; we booked Y fare on way back. When she went to make the changes, my refundable fare was changed to non-refundable. She figured this out but had to transfer me to another agent to complete the changes but I got cut off during transfer. Hours later when I was able to talk to,someone, I was told that my fare was not-refundable and there was nothing that could be done. Fortunately, I was then transferred to a ticketing agent who was extremely helpful and sorted everything out. So in the end, my fare remains refundable; Premium outbound; F inbound after 15k mileage upgrade; inbound Premium seats refunded.