Note that the eCredits generally don't seem to show in the eCredits listings in your Profile. If you didn't save a copy when you made the change, you can find the 006 eCredit number in either the Flight Receipt email for the flight change, or online within the receipt for the flight change in your wallet My Receipts listings -- https://www.delta.com/mydelta/walletMyReceiptSummary (it will be listed as "Remaining eCredit Balance").