Joomla 1.6 officially released, there have been a lot of questions as to how to upgrade or migrate from 1.5 to Joomla 1.6. This guide will take you step-by-step through the general procedure of how to upgrade from 1.5 to Joomla 1.6.
The joomla code is the same from Joomla 1.5, much of it has been written from the ground up, and the changes are comparable to the changes from Joomla 1.0 to 1.5. Because the changes from Joomla 1.5 to 1.6 are so large and because of the massive effort put into getting Joomla 1.6 to where it is today, there is no core upgrade path, this is indeed a migration. In planned future releases of Joomla (which will be released every 6 months), such as Joomla 1.7, 1.8, etc, the changes from version to version will be more incremental and a core upgrade path is planned. Now that Joomla 1.6 is finally here and stable, a community initiative led by the developers of Joomla is turning towards jUpgrade (a 3rd party Joomla extension on the JED originally developed by Matias Aguirre) for help and to help.
Please review the technical requirements for Joomla! 1.6
Please review the requirements for jUpgrade
Here is a good video from the youtube Joomla! Channel, describing the use of the component.