Just after I performed the Maverick (10.9) system update on my MacBook Air, I found that some of my Pages documents would not open.  I was told in a dialog box on my computer screen  that I needed a new Pages version and I was directed to the App Store.  The App Store showed that I had the current version.   I discovered that Maverick did update my older versions of iLife programs but failed to delete the former software.  It seems that some of my former documents were saved with the current new Pages version.  All of the iWork applications were now in the Applications folder as individual programs and not in the iWork ’09 folder.  Just to cover any future problems, I copied the iWork ’09 folder into my airport backup drive and then deleted that folder from my Applications folder and rebooted the computer.  Now there is no problem.  All my old Pages documents open up properly.