Even if there are 3 D1 seats left, it's unlikely they would release them for upgrade before the gate. Which is unfortunate for people who want to use miles to upgrade, since per the rules (and sometimes the rules can be broken) mileage upgrade requests do not get transferred to the airport upgrade list. Either you have to clear in advance or not at all, unless you find employees willing to break the rules for you and transfer your upgrade request to the airport list.