You were smart - or got stuck buying that as the lowest available fare class. That put you ahead of ahead of all Plats on H, Q, K, L, U, T.

For most routes my experience suggests DL offers the upgrade confirmations as you propose. The clear exceptions have been some 757 routes (where they might sub a 16F cabin for a 26F cabin). In those cases I have seen 7-9 people called to the gate a moment before boarding starts.