I have several CO flights in the next two weeks already booked, amounting to about 6,000 miles. I'm considering changing my FF allegiance to Delta for the coming year. The first of these flights is in 4 days, on the 6th.

My question is - if I join Delta SkyMiles online now, and switch my FF number on the already booked reservations, either online or at the gate, should I have any problems getting the miles for these upcoming flights credited to my new SkyMiles account instead of my OnePass?

Thanks!