1 Leptin is a powerful and influential hormone produced by your fat cells. 2  It plays a key role in regulating energy intake and energy expenditure.
(Upgrades are not available on fares in the L, T, and U range, however, which are the most deeply discounted.) To calculate the price of an upgrade, Delta charges a dollar amount per 500 miles. $50: Up to 500 miles. $75: 501-1000 miles. $100: 1001-1500 miles.
Step 2. Choose View Upgrade Options on the self check-in kiosk at the airport on the day of your flight -- or, if a Delta representative assists you, ask him to go through upgrade options with you. If you wish to have a confirmed upgrade on your day of travel, you can pay either with SkyMiles or a credit card.