It will not allow you to update the old version unless you uninstall the new versions first. Just did this on 2 MBP's I bought. Installed iWork 09 and tried to update. It told my I had a more current version and would not allow the update. Removed the new versions of iWork and update worked no problem.
Had to get the updates directly from Apple. Just do a Google search for the iWork 9.3 update which is the last update to iWork 09 as far as I know. Someone will let you know if I am not correct about that update version. This applies to iLife programs as well.
You can then just reinstall the new versions back if you want them.