SFO-JFK. Booked with miles. Tried SDC on app. I get "PNR is not eligible for Same Day Change". On the web, only SDS is available. Called, they can only do standby even though main cabin seats are open. Agent tells me has to be same class of seats. Okay. Did the standby. Got C+ middle seat. When I boarded, they switched me to the aisle with middle seat open. Fantastic! Wish I could have used a RUC for one of the D1 seats but sometimes, you just can't have it all.