Ich sehe da kein Problem.... ich würde sogar eine überinstallation wagen...
Problem
When installing a version of Lotus Domino that is lower (earlier) than the one currently installed, you receive the error message:
Unable to install Lotus Domino over Lotus Domino: Installed product has newer version.
NOTE: This error message prevents you from downgrading Domino as prescribed in technote 1219581.
Cause
Implemented in Domino 8.x is a Server Version compare during Server Install. This check verifies that the version you are installing is greater than what is already installed for compatibility reasons.
Resolving the problem
In order to downgrade, you should:
1. Stop the Domino server.
2. Uninstall Domino from the operating system. By default this will leave Domino's program directory intact.
3. Install the older version Domino Software, pointing to the correct Program and Data directories.