I was in the same situation and I called Reservations and explained to them.
I had to be put on hold for a while as they had to manually reset the higher
rates and add my EC# to the reservation. So it can be done.
Originally Posted by JT_BOS
Just found out I could not add my NEC # to an existing reservation. The only way is to cancel it and book a new one. Of course, the daily rate is more than $10 more expensive now, which I am unwilling to pay.
I do not understand the logic behind this. Why would this make a difference? The airlines allow one to add frequent flyer # after the ticket is purchased.