While I'm not advocating trying this, I have had FCM offers paid in miles successfully refunded to me when I voluntarily cancelled a reservation: Back in October I booked a 4 segment trip and applied RUCs. Only 3/4 cleared instantly and the one that was waitlisted had a very reasonable upgrade offer with miles. I took the offer so as to have all segments in F and not have to worry about it. I ended up cancelling the trip and fully expected to lose the miles, but to my surprise they got refunded.

However now that I think about it, I believe the trip itself was an award booking, originally purchased with miles, so no money was ever exchanged. That could be a factor.

Anyway, I would suggest just leaving things as they are if you have already purchased the upgrade. Enjoy the earlier boarding of C+ and free drinks.