Upgrading a NW using OP miles
 

{Sorry that this is cross-posted--this is my first time posting and accidently posted this to the NW board first. It won't happen again.}

I am a CO Silver in Rome. I have the second part of my NW "H" ticket (FCO-DTW-CLE) for my return mid-August. I would love to upgrade the FCO-DTW, a 10-hour flight. I was told by the CO agent (in Rome)that I couldn't upgrade using miles because I didn't buy the ticket through CO. Is this the common experience? Anyone have any helpful suggestion?