Originally Posted by
MSPeconomist
According to DL rules, you cannot change the FF number associated with (any flight on) an itinerary after you have boarded the first segment. Hence, this will be more of a problem if your status changes in the middle of a trip. In this case, the remove and reenter the FF number trick is not a do it yourself project. You'll need to talk to a good agent.
I recently had this issue when I went from FB Silver to Gold while in transit to and from Tokyo. When I wanted to use the JFK Sky Club, the lady just verified the actual status with me (it showed up as Silver on the PNR) and I was on my way to the showers.