The Magento 1.3 to 126.96.36.199 upgrade is quite complex, as there are many changes to the database structure and file system.
This is the cleanest way to upgrade a Magento 1.3 installation to Magento 1.4 (specifically 188.8.131.52). This will ensure that you keep all orders, customers and other settings intact and this method provides the quickest database structure upgrade (upgrading using Magento Connect is NOT advised). It also allows you to run Magento 1.3 and 1.4 versions side by side, keeping your 1.3 production version live and operational while you work on the new 1.4 version compatibility changes behind the scenes.
You will have to reinstall and configure any modules / extensions of course, as well as merge any custom changes. If you use PayPal, you will need to make a few changes, such as re-enter your account settings and activate any new features in the new PayPal interface).
These instructions are geared for Create Hosting customers on the Magento Hosting Plan which uses Plesk Parallels Panel.-- removed --