According to Bleeding Cool, DC Comics have asked retailers not to release two comics, Superman #14 and Supergirl #33, with the covers they were initially shipped with.
The change comes because of a change in story content, according to the article. For Superman #14, the story was not part of the Year of the Villain: Dark Gifts storyline, and adds a tease for the Legion appearing in the issue.
With Supergirl #33, the story retains its Year of the Villain branding, but also adds the Legion element as a tease.
Replacement copies will be provided to retailers for free.
Here are the initial two covers:
And the new covers: