I have "non refundable" F tickets next week for Maui routing through SEA...I tried REAL HARD to switch routing through HNL...At one time there was F seats available (for change fees and $ fare increase that I was willing to pay) BUT those seats would have been "refundable" so they couldn't do the exchange...The way it was explained to me they couldn't (or didn't want to) "track" the original "non refundable" portion on the new "refundable" ticket so I could not change to the DC-10 Comfortable flights!

I'm not sure if this is close to your situation, but it made no sense to me.