If you "upgraded" to Pages 5 you now have 2 versions of Pages on your Mac:
Pages 5 in your Applications folder.
Pages '09 in your Applications/iWork folder.
Pages '09 can not open Pages 5 files and the Mac will tell you that you need to update.
Pages 5 will open and alter/damage Pages '09 files which, once converted and saved, can not be opened again in Pages '09.
Pages 5 can not open files earlier than Pages '09, eg Pages '08 and Pages '06.
Files passed through iCloud will automatically sync back and also be in the Pages 5 format.
All the files have the same .pages extension but are largely incompatible with each other.
Apple has removed over 90+ features from Pages 5 so it is largely a downgrade:
http://www.freeforum101.com/iworktipsntrick/viewforum.php?f=22&mforum=iworktipsn trick
If you do not want to use Pages 5, trash/archive it after Exporting any Pages 5 files to either Pages '09 or Word docx.
Turn off iCloud syncing for Pages so that your files are not converted, turn off Auto update in backbround, in System Preferences > Software Update, then get on with your work.
Peter