According to the Aperture User Guide, you should be able to control-click a project and choose “Update Previews” from the contextual menu. Oddly, my instance of Aperture (3.2.2) doesn't have this option in the contextual menu, no matter how many times I control click. The workaround is to select a project and then choose “Update Previews for Project” from the Photos menu on the menu bar.

Note there's a minor GUI inconsistency: if you choose a photo in a project and then deselect all photos the menu item will read “Update Previews” rather than “Update Previews for Project”. It will still work the same.