Last week, Delta Air Lines announced that one of the qualifications for its highest published elite tier, Diamond Medallion, would require spending at least $15,000 on Delta travel – or spending $250,000 annually on Delta Amex, a change from the previously required $25,000 on the credit card. The change is effective next year for 2019 status.The secret Diamond Medallion Facebook page and other sites page lit up immediately with most commenters threatening to switch carriers or credit cards....