The change comes after a passenger, Dr. David Dao, was dragged from a fully-booked United Express flight in Chicago because he refused to give up his seat to make room for crew members.
Cellphone video of the incident sparked widespread outrage and created a public-relations nightmare for United.
VIDEO: United Airlines passenger dragged off overcrowded flight
Under the change outlined in an internal email dated April 14, a crew member must make must-ride bookings at least 60 minutes prior to departure.
Previously, the crew could be booked until the time of departure.
United spokeswoman Maggie Schmerin said in an email Sunday that the change is an initial step in a review of policies, and it's meant to ensure that situations like Dao's never happen again.
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