Yes, you can almost always upgrade to Delta One (Delta’s premier business class) after your initial purchase, provided there are seats available in the cabin. The most common way to do this is via the "Fly Delta" app or the website under the "Manage My Trip" section. Delta often presents "Post-Purchase Upgrades" which can be paid for with either cash or SkyMiles. These offers are dynamic and can change daily based on seat availability and time remaining until departure. If you have a high-level Medallion status, you may be eligible to use a Global Upgrade Certificate (GUC) to move into Delta One on international routes, though these are subject to specific "clearance" rules and seat inventory. It is important to note that if you originally purchased a "Basic Economy" ticket, you are generally ineligible for any upgrades, even for a fee. If you are already at the airport, you can ask a gate agent about "last-minute" upgrades, though these are increasingly rare as Delta aims to sell those seats through its app beforehand. Always check the fare difference versus the upgrade offer, as sometimes re-faring the ticket is actually cheaper than the one-click upgrade button.