Hi Robert Gill
Once the older Copilot/Legacy app has been replaced or uninstalled, Microsoft does not provide a supported way to roll back to the previous version. Recent Copilot updates have moved users to the new app experience, and rollback to the pre-migration app is generally not supported.
Since your iPhone X on iOS 16 should still meet the documented minimum requirement for the current Copilot app, try downloading Microsoft Copilot again from the App Store and sign in with the same account you used previously.
If the App Store no longer offers a compatible version for your device, you'll likely need to use Copilot via a web browser instead, as there is no documented method to restore the retired Legacy app.