You have 2 versions of Pages on your Mac.
Pages 5.2 is in your Applications folder.
Pages '09/'08 is in your Applications/iWork folder.
You are alternately opening the wrong versions.
Pages '09/'08 can not open Pages 5 files and you will get the warning that you need a newer version.
Pages 5.2 can open Pages '09 files but may damage/alter them. It can not open Pages '08 files at all.
Older versions of Pages 5 can not open files from later versions of Pages 5.
Once opened and saved in Pages 5 the Pages '09 files can not be opened in Pages '09.
Anything that is saved to iCloud and opened in a newer version of Pages is also converted to Pages 5 files.
All Pages files no matter what version and incompatibility have the same extension .pages.
Pages 5 files are now only compatible with themselves on a very restricted set of hardware, software and Operating Systems and will not transfer correctly on any other server software than iCloud.
Apple has removed almost 100 features from Pages 5 and added many bugs:
You have two versions of Pages on your Mac & are trying to open Pages 5 documents with Pages 4. Are you launching Pages from an icon in your Dock? Installing the update does not change the Dock icons & it does not remove the older versions. Go to your Applications folder & launch the new Pages from there then right-click on the Dock icon & select Options > Keep in Dock.
Installing new versions of iWork has never replaced existing versions. This is nothing new. The only thing new is that the '09 apps are moved to an iWork '09 folder if they were purchased from the Mac App Store. If they were installed from disk they should already be in a separate folder in Applications.