You cannot upgrade a CO flight with a BusinessFirst cabin at the airport (I'm not actually sure about other flights). If you are able to upgrade a NW flight at the airport, you probably won't be able to do it without a CO certificate in hand. You might be able to get this issued by a CO agent at the airport if you set it up beforehand over the phone.

There is a very strong possiblity that your CO miles will not post to your account within 48 hours.

It is possible to have a negative balance in your Onepass account. I was able to claim an award when I didn't have enough miles but knew some would be posting. But that was only few thousand miles. Dpending on how many miles short you are, you may be able to get CO to do the upgrade even though you don't have enough miles.

YOu also need to be aware that not all fares are upgradeable.