Only at time of booking and only for flights with US 50.
Not sure what you mean by this. As of June 20th, 2017, you can pay for change fees (and other fees) using miles:
Starting June 20th, 2017, SkyMiles Members can use their miles instead of money for even more products and services when calling Delta Reservations, giving Members more flexibility and options than ever before.
Members can now use their miles for:
To take advantage of these options, contact Delta Reservations at 1-800-323-2323.
source: https://www.delta.com/content/www/en_US/skymiles/program-resources/news-and-updates.html