AFAIK all domestic flights have $200 change fees, assuming not BE and not fully refundable or changeable without penalty. However, to verify this you would need to look at the fare rules, which someone here might be able to find (for instance on EF) if you tell us the exact fare code(s). Also, DL could market some short international segments as FC, such as perhaps Canada transborder flights.

Does the approximate (does this mean it's not a round number) $500 fee include a fare difference? That would be my guess. Try to price the flights you want as a new ticket to check the fare difference. Changing before you fly the outbound normally means that the entire ticket is repriced, which is subject to current fares, current fare class availability, and any advanced purchase requirements for the (new) fares.