On Delta Air Lines, you generally cannot get a "complimentary" Medallion upgrade from Comfort Plus to Premium Select on international long-haul flights. Delta's policy treats Premium Select (their international Premium Economy cabin) as a completely separate class of service rather than just a "seat preference" like Comfort Plus. Complimentary upgrades for elite members are usually limited to domestic routes and short-haul international flights (e.g., to Canada, Mexico, or the Caribbean). For long-haul transoceanic flights, the only way to move from Comfort Plus to Premium Select is to pay for the upgrade using cash or SkyMiles, or to use a Global Upgrade Certificate (GUC) if you are a Diamond Medallion member. However, Delta frequently offers "post-purchase" upgrade deals in their mobile app that can be quite reasonable if the cabin isn't full. It is always worth checking the "Select Seat" screen in the app after booking to see if a discounted offer is available to "buy up" to the superior seat and enhanced meal service of Premium Select.