The option is subject to availability and only possible for flights operated by Delta, rather than by its partner carriers.
Atlanta-based Delta said it plans to make the mileage upgrades possible via its app in early 2019.
It's part of a shift by Delta away from giving out free upgrades to frequent fliers based on their status.
Airlines also get a benefit when passengers burn their miles and use the frequent flier program more.
The shift to a bigger focus on selling first class seats has irked frequent fliers who see free upgrades as one of their biggest perks in return for their loyalty. When more first class seats are sold, it decreases the chances for free upgrades based on status.
Separately, Delta is testing the sale of basic economy tickets for miles on certain routes, including flights from Charleston to the rest of the United States and Canada, and travel between Minneapolis and Phoenix.
However, basic economy tickets cannot be upgraded or changed and do not get a seat assignment until after check-in.
Delta often aims to "upsell" passengers from basic economy to a pricier fare.