About a year ago I was flying LGA-SEA on a through flight which made a stop in PIT. It took a mechanical at LGA, and all they had for backup was an A-319 Shuttle plane, which is single class. As a result, those of us in First had to get separate boarding cards for each leg. The flights posted with the same number, but different city pairs. The LGA-PIT segment credited at the 500 mile minimum, and I got actual miles flown on the PIT to SEA (which was on an A-320, configured normally)portion.