The standard new card bonus for the Delta Reserve card is 10,000 MQM. But if you upgrade from Delta Platinum to Delta Reserve you only get the difference between the signup bonus for Platinum - 5,000 MQM - and the 10,000 MQM bonus for the Reserve card, netting you only 5,000 MQM. The advantage to upgrading is that the annual fee for the Platinum card is applied toward the Reserve card.

If you want to get the whole 10,000 MQM you can't upgrade. You get an entirely new card. You have to pay the whole annual fee, but you get a new and full 10,000 MQM.