Updated August 22, 2019: Superman #15 and Supergirl #34 have been pushed back one week on DC's release schedule, now timed for a September 18 release.
The publisher didn't specify a reason for this schedule change, however the previous issues of both titles were recalled with the revised Superman #14 coming out August 28, and Supergirl #33 coming out September 11.
DC attributed the recalls to "the covers of both issues ultimately did not reflect the contents of the stories." The publisher hasn't specified the reason for this new schedule change.
Original Story from August 10, 2019: DC is asking retailers to destroy all copies of August 14's Superman #14 and Supergirl #33, with replacement copies coming at a later date.
"Due to advance production schedules, the covers of both issues ultimately did not reflect the contents of the stories," DC told retailers. "Replacement copies with new covers will be provided at no cost to retailers."
The original covers are branded as "Year of the Villain" tie-in issues, however the new covers will not carry that branding according to the publisher.
DC is also changing out the cover art - with Superman #14's original Ivan Reis/Joe Prado art replaced by a new piece from the duo, and Supergirl #33's Eduardo Pansica art replaced by a new piece from Kevin Maguire (as yet unrevealed).
With this, the new on-sale date of Superman #14 will be August 28 (two weeks later than solicited), and Supergirl #33 will be September 4 (three weeks later than solicited).
Superman #14 is the advertised return of the Legion of Superheroes, and the new cover features a Legion logo in the bottom right.