There was a schedule change, I made the flt. change I wanted for free.
But knowing what the rules are will be useful in the future..
But for DL Vacations, DL does not see what I actually paid [I bought a package], so knowing what I would have paid on the date of purchase would come into play for computing "fare difference".
But for the new flight that you might want to to change to at some time in the future, the thing that matters is the price at that moment in the future, not any historical price