Originally Posted by gooselee

Often DL will try to upsell you from Main Cabin to C+ or FC. If so, it'll appear as a banner ad at the top of your reservation screen (when you actually access the trip from My Trips) - some language like "Upgrade to First Class for $xxx per passenger entire trip"

However, sometimes DL won't make those overtures. In that case, you can always call in and an agent will happily quote you the upgrade price at that time. I've never been charged a change fee to upgrade from coach to FC when I call in to do that - just the fare difference.