Yes, you can upgrade to Delta Comfort+ for free, but it is primarily a benefit reserved for SkyMiles Medallion Members. All elite tiers (Silver, Gold, Platinum, and Diamond) are eligible for "Unlimited Complimentary Upgrades" to Comfort+ on flights within the U.S., Canada, Latin America, and the Caribbean. The "window" for the upgrade depends on your tier: Diamond and Platinum members can often upgrade immediately after booking, while Gold and Silver members are added to the list closer to departure. If you are not an elite member, you can occasionally get a free upgrade if the "Main Cabin" is overbooked and the airline needs to move passengers up for operational reasons, but this is rare. Another path is holding a Delta SkyMiles Reserve Card, which puts you on the upgrade list behind the Medallion members.