Columnist Scott McCartney explains the changes Delta Air Lines is planning to make to woo and retain elite travelers. For example, the carrier will allow frequent fliers to roll over their miles from year to year. The move is aimed at discouraging travelers from using competing airlines. "Many customers split their share," said Jeff Robertson, vice president of loyalty programs at Delta. "We see customers who midyear hit 51,000 miles [qualifying for Delta's gold status] and never fly us again the rest of the year. But we know they are still flying."