If I've read correctly, the OP was considering booking an F out, Y back redemption under the promotion, which is only 2/3 the miles required for a one-way in F, and 1/2 the miles required for his current, non-promotional redemption roundtrip in J. I also understood him to be saying that he'd cancel the outbound of the current redemption in J, turning the current roundtrip redemption into a one-way redemption.

The question marks for me are whether, if his roundtrip in J is indeed booked as a roundtrip redemption (as opposed to two one-ways, which is what I generally do in order to retain flexibility after departure), can you simply cancel the outbound of a roundtrip redemption and get half the miles and appropriate taxes back, or would you have to cancel the whole redemption and re-book?