I originally built the Classic Cougar Community as a Joomla! 1.0.x site back in 2007. When Joomla! 1.5.x debuted, the client had me migrate the site to the new version. Now, Joomla! is up to version 1.7.x, but currently there is not a simple, straight upgrade path from Joomla! 1.5.x to 1.7.x; it’s a migration.
The success he’s experienced with other WordPress sites led my client to opt for migrating the site to WordPress, a platform that in my experience is much easier–and less costly–to maintain and upgrade, rather than to migrate it to the newest version of Joomla!.