I can make a flight change with NO fees, except I must pay to reflect the difference in fares--what base is DL using? The price for my flight vs. the price for the flight I want to change to on the change dateor the price I paid when I purchased my ticket vs. the price I want to change to on the change date.
If my flight is part of a DL Vacation package how do they know how much I paid?
PS: I want to change from a d1 later to a d1 earlier on the same day.
You'll have to contact DVC not DL to change your flights as not all DL tickets are published fares.
It's of course based on what you paid.