I recently purcharsed a ticket using an ECV for $ off, which I received for being voluntarilt bumped. Anyhow, I had the option of upgrading the tickets on nwa.com. It would not go through however, stating that I did not have enough miles to upgrade. I did have enoguh miles, so I called the gold/plat line and the rep upgraded me using the miles as I had intended to do.
So there are some success stories around using miles to upgrade a ticket purchased with certificates.
Benz3000