Joomla! 1.5 does not provide an upgrade path from earlier versions. Converting an older site to a Joomla! 1.5 site requires creation of a new empty site using Joomla! 1.5 and then populating the new site with the content from the old site. This migration of content is not a one-to-one process and involves conversions and modifications to the content dump.
There are two ways to perform the migration:
