I've encountered this as well with a couple clients of mine (I am an independent Apple Consultant). Simply restarting did not work for me (but might solve the problem depending on what the issue is in your particular case so it's always worth a shot). In the cases I've come across, we tried trashing the newer version of Pages and reinstalling from the AppStore (you'll likely need to reboot after trashing in order for the AppStore to recognize that the newest version is no longer installed), and we trashed the preferences file for Pages at ~/Library/Application Support/Containers/com.apple.pages.plist I'm guessing that it is the preference file which is the actual fix, but as I did not have time to try both I can't say for sure. It appears as if the Pages 5.0 or 5.1 preference file became corrupt after installing the 5.2 update.
Having the older Pages versions installed within the iWork 09 folder did not appear to be part of the problem as both versions coexisted just fine in the cases I'm citing.