Hi,
When an MSI package is launched Windows Installer checks if another version is already installed and if a newer version is found it displays an error message and stops the installation.
If you want to install an old product over a newer one, you must first uninstall the new one.
A workaround for installing an old product over a newer one is described in the
uninstalling version if trying to install older version thread, but keep in mind that this is not the right approach.
Also, you can keep both products installed by checking the "Allow side by side installs of different product versions" checkbox in the Upgrades page.
Best regards,
Eusebiu