When uploading a story, you can now swipe up on the screen and be shown a reel of all the photos you've taken in the past week (as opposed to those taken in the past 24 hours).*
Even after its public release, this feature may not change much about the Stories experience. As one user noted, it has always been easy to post older photos as Stories by screenshotting or editing them.*
The trick is to just slightly edit a photo and it becomes "new" and you can post