Unless you're an elite, most FF programs charge fees to change or cancel award tickets. For DL, it's $150 per ticket unless the miles came from a PM or DM account, meaning roughly 75,000 or more status miles per year. On DL, you cannot change or cancel an award ticket within three days of departure without forfeiting ALL of the miles used to pay for the ticket, although IIRC taxes and fees are refunded, perhaps in the form of a DL credit. Of course, you can only change dates if award inventory is available for the new flights.