Originally Posted by
boat9781
Well to follow-up, I called Delta, and the agent charged me $150 to refund the 'existing miles not used.' Not really sure what that means, but he said it was the best option available to me so I accepted. The miles have yet to show up in my account, but I guess that just takes time.
This is BAD news. There will not be any "refund of existing miles not used" since the number of miles needed for what you already flew is the same as the number of miles needed for your round trip. In fact, you already flew part of the second half of the ticket (PHL-SNA) so I am sure even if there was a chance to get a refund of some miles with DL (if they offered one way awards) it would not have applied to you anyway.
I would call back and try to clarify but I hope you get to use your miles - even with a $150 x 2 change fee it would be cheaper than a one-way ticket.