Originally Posted by D582

The previous set of these fares were dual-inventory fares that booked into B but required inventory in a lower booking class as well. BLTFFKLT likely required K and BLTFFTLT likely required T. The new versions of these fares are single inventory booking directly into the lower RBD (i.e. K and T directly). All of these fares are fare-by-rule based on another lower fare with an upcharge applied. When changing these after departure, it will use fares in effect at time of ticketing, but since they are dual inventory fares, you need the same lower fare class available to keep the same fare (notwithstanding other things such as day of week etc). So an ad-col may be because you need to move to a higher fare.