First of all, you need to backup all your company databases followed by an upgrade of your PostgreSQL installation. There are plenty of places on the web to explain how to do that. The high-level process is to install the two versions in parallel and run the pg_upgradecluster command.
When the technical upgrade has succeeded, however, you're not ready to see the performance improvements promised by the 8.4+ versions of PostgreSQL with respect to the menu-generation. This is because the database doesn't automatically use the new 8.4+ code definitions.