If it's an award ticket, normally seats must be available in award inventory at the same level (or you will be charged more miles), which frequently happens as the flight date approaches. If you're not PM or DM, a change fee of $150 is charged. However, if there's been a schedule change, you should be able to call and change for free, frequently even if award seats aren't available on the flight you want. The same rules should apply if there's a weather waver in effect. If not and you're concerned about weather delays and a potential missed connection, call DL and plead your case.