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Can I revert back to Joomla! 1.7 or Joomla! 1.5 after upgrading to Joomla! 2.5?

That won't be easy and will probably require an awful lot of custom work. Our suggestion is to make an informed decision about upgrading to Joomla! 2.5 and at all times make sure you have a recent and tested backup before starting the update process.

More information about Joomla! 2.5