Delta's new rules come into effect for travel on or after May 16th 2016. You will no longer be able to select Delta Comfort + seats for free at booking time if your Medallion level allows you to do so. However you can do so after the booking is complete by selecting the box (which is NOT selected by default) to upgrade for free to Delta Comfort + at time of booking. If you forget to do so during booking, you can go back in and select it by managing upgrades. I just wanted to alert people because I made the mistake of not checking the box at first and then couldn't figure out what it wanted to charge me when I knew that it wasn't supposed to. More information is here