One of the problems when you upgrade to Joomla 1.5 is that your old urls will be giving 404 errors which is bad, not only because you are losing visitors but also for your site SEO - Google, Yahoo and those other search engines don't like 404 errors when ranking your web site.

I've come across a great component to handle this once it is set up but the set up can be a bit tricky.

With PhocSEF you replace your old pages with a redirect to your new pages as visitors try the old pages. Depending on the size of your site you can redirect to the replacement new pages or you can redirect to a specific page, e.g. a notification of the change or your home page. 

Installation of PhocaSEF:

The first part is a normal component installation, however:

• You need to add a bit of code to your error.php page in the libraries/joomla/error/error.php page (not the same one as previously mentioned)
  Add:

//PHOCAEDIT
if ($code = '404') {
include_once( JPATH_ADMINISTRATOR.DS.'components'.DS.'com_phocasef'.DS.'helpers'.DS.'phocasef.php' );
PhocaSefHelper::createUrlEntry();
}
//PHOCAEDIT

before

$reference = & JError::raise(E_ERROR, $code, $msg, $info, true);

in about line 171.

• And it doesn't automatically update your database. So you have to do a sql dump of the contents of install.bak.The install.bak file is in the folder administrator/components/com_phocasef. Remember to change the #__ to jos_ (or your database prefix) in the sql dump file

Once this is done you can go to the component and manually add your pages or you can wait for the 404s to be listed automatically and then edit and or publish them. Over time your should pick up all your 404 errors

Download PhocaSEF