If your old iWork '09 applications were installed from the retail iWork '09 DVD, then you will have to reinstall them, and then apply the Apple iWork 9.3 update, for El Capitan compatibility. If those iWork '09 applications were purchased from the OS X App Store, then only those apps can be moved/restored to your current machine. Never apply the Apple iWork 9.3 update to OS X App Store sourced iWork '09 applications. You will need an external DVD player to reinstall iWork '09 DVD applications.
You cannot rent iWork '09 applications, and these applications are no longer available from Apple. If you attempt to download Pages, Numbers, and Keynote from the App Store, they will be the latest updates, and not iWork '09 applications.
I know that Pages '09 v4.3 (last revision) has begun to exhibit some lost features in Yosemite and El Capitan. Apple has zero focus on maintaining foreward OS X compatibility with the retired iWork '09 applications.
- Sharing menu
Send via Mail items no longer work
- Mail Merge
Some Mail merge interface menus are missing that are shown in the Pages '09 User Guide.