OP:

When you do not fly the flight in question you are left with a DL credit of the paid amount minus $200 to be used on a DL flight (in almost all domestic tickets). There is no difference between canceling and modifying.

There is a reasonable chance DL would waive the $200 fee given the circumstances (though this is not something they owe you).

The real problem is you now want to apply that credit to a flight that is more expensive by $600. That is the issue in play.

Your only real hope for saving cash is to find a cheaper alternative to reduce that $600 fare difference, while also asking DL if they could waive the $200 change fee.

EDIT: I think you are saying it is the exact same flight that is now $600 more expensive. That does not matter at all. All that matters is that you have $1250 DL credit to use for the same pax within the next year (+--).