What if I don't ?
What will happen if I don't migrate my Joomla 1.5 site to Joomla 2.5?
After April 2012, your Joomla 1.5 site will continue to function normally. There will be no change in functionality or loss of service. However there will be no ongoing support from the Joomla development team, and the site will no longer be considered secure.
