From Joomla! Documentation
< Extension Installer | TriggersRevision as of 22:29, 24 September 2008 by Pasamio
The onBeforeExtensionUpdate trigger occurs before the update of an extension. It has the following parameters:
The type of extension that is being installed, for example 'component' or 'plugin'
A copy of the manifest of the extension about to be installed. This is only populated when the method is 'install'.