It's already live I think. It is for me anyhow. I bought the ride up front for 4 segments w/ miles at time of purchase, now it's trying to get me buy C+ on the CRJ leg(s)
It's rare that I'm on cheaper fares booked a week in advance like this one, and have rarely seen mileage upgrade availability even on some of the higher fares. Was actually surprised to see it offered now.