This situation is not unique to DL. I'd say you've been lucky over time to only have been paying the change fee and not a additional collection for a new fare. Meaning, your fare basis has always been available when you changed to the new flights. When it's not available, you will have to pay extra for a higher fare, thus the 503 bucks they wanted in addition to the 75.00usd for the change fee.
Since your flight is a few months away yet, you could try what Silent Bob said and call back to get a different agent until you get a $$ amount that you like better. Fares change constantly, people sometimes actually call to cancel their seat so maybe the lower fare will open up again.
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