Preview maintenance can be rescheduled for projects in which automatic preview maintenance is turned off.
In the Library inspector, select the project whose JPEG previews you want to start updating again, then choose Photos > Update Previews for Project.
Note: Each time you open Aperture, it checks for out-of-date previews in those projects whose automatic preview maintenance option is turned on and then schedules the previews for regeneration. If you don’t want previews updated automatically, turn off automatic preview maintenance for your projects. For more information, see Automatically manage previews.