All CW fares can be UuAed including I class.

It is probably a reward availability issue. Have you checked that F rewards are availalbe for your flight by doing a dummy booking for a straight F reward on ba.com?

if there are F reward seats available on your flight, it might not be allowing you to upgrade in mmb if you have domestic flights or other carriers on your booking. In that case you'll need to call BAEC to get the UuA put through.