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Do not do it on a live system. Do it on a test copy of your system first.
Last time I upgraded it took me days to repair the damage. If you have made any alterations to the design or code, or email templates, many of them will be broken in major ways. Not good if your customers can’t access the site properly while that is happening.
I haven’t customized it at all, it’s just that my hosting provider, Media Temple, has a one-click installation feature of Magento, but it is only v1.0. After installing v1.0 I went to manage extensions, put in the key “magento-core/Mage_All_Latest” provided by JHLC and got this error on my site.. please click on the website icon under my user name