Long discussion and Wiki FAQ's here --
http://www.flyertalk.com/forum/delta...16-2016-a.html
Domestic C+ upgrades are handled like FC upgrades now. You need to be sure you have the checkbox selected that you want the C+ upgrade. Upgrades can happen anytime from 24 hour mark to at the gate for a Silver (similar to FC). You can't select a seat until you get the upgrade.