Originally Posted by
dwbf11
Thanks all for the replies! And yes, from my research on this question I also read that switching mid-itinerary may be problematic, so I'll run the whole trip through on the FB FF#. As it's a short-haul trip having the SM benefits is of little import here. Much better for me to slum it for an hour than to lose the miles

Exactly. DL won't let you change a FF number on an itinerary after the first flight has been flown (or maybe the rule is checked in, boarded, etc., but you get the picture. It's all or nothing.