Last update: September 02, 2015
This is the most comprehensive description of the Magento 1.3.x, 1.4.x, 1.5.x, 1.6.x, 1.7.x, 1.8.x and 1.9.x (1.9.2.1) upgrade process.
Additionally It contains step-by-step instructions and troubleshooting information.
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