As I'm soon to drop to Platinum as a result of the Amex waiver increase (well, technically because I didn't reach the MQD threshold), I was considering upgrading my ticket with miles at purchase. Higher MQMs and new complimentary upgrade calculus as a Platinum. The problem is that I also wanted to use gift cards to pay for a portion of the ticket, but Delta.com doesn't allow the use of gift cards when upgrading with miles. I found it interesting to note that the number of miles each way required to upgrade my fare (LGA - FLL RT) was 20,000 each way prior to purchase. Once I bought the ticket and went back to select seats, the outbound portion required 40,000 miles by itself and the return portion required 75,000 miles to upgrade. In short 40,000 miles to upgrade pre-purchase and 115,000 after purchase.

Is there a revenue management reason for the discrepancy or is this just the limitations of Delta's IT on the back end?

And why doesn't Delta allow gift card usage when upgrading with miles?