The template I'm going to demonstrate is Versatility4 which you can get from https://rockettheme.com. RocketTheme does have an updated version of this, but it's a good example of a fairly complex template for this tutorial. Still, it's worth checking to see whether your template provider has a Joomla 2.5 version already created for you. It will save you from going through this complete tutorial

The process of moving your template involves several parts:

1. Move your main site to Joomla 2.5.
2. Create a new template folder for your new Joomla 2.5 template.
3. Move over the files and folders from your Joomla 1.5 template to your 2.5 template. Except for index.php and templateDetails.xml.
4. Migrate the code in the templateDetails.xml file.
5. Migrate the code in the index.php file.
6. Discover and publish your template in the Joomla 2.5 administrator area.

That's the general process that we're going to use. Here are the specific steps you'll follow:

Before you start, use jUpgrade or SPUpgrade to create a new version of your site in Joomla 2.5. You can do this using our 1.5 to 2.5 update tutorials for jUpgrade and SPUpgrade. Neither of those will modify or upgrde the templates from 1.5 to 2.5, although the directory may (or may not) be copied to the upgrade destination. We're going to assume it wasn't copied.

• Access your Joomla site files and go to the /templates/ directory.
• Copy the entire /atomic/ directory and give this new directory the name /atomicVersatility/. The Atomic template is extremely basic, and was really intended for purposes like this. You can use this process with other base templates as well, but this one is free and already installed.

• Go inside the /templates/atomicVersatility/ folder.
• Delete or rename the template.css file. We want to replace this and don't want to use any of the Atomic CSS. You probably won't need to change the template_ie.css and template_rtl.css files. 

• Open /templates/atomicVesatility/templateDetails.xml, and change the entry to match the name of the template folder. It is case-sensitive so make sure you type accurately.
• Change the other elements as necessary. For instance, you might want to modify the creation date and version number and the description.

• Go inside the old Joomla 1.5 /templates/yourtemplatename/ folder and open the original templateDetails.xml file.
• Find the positions section and copy it to your clipboard.
• Go inside the /templates/atomicVersatility/ folder and open the new templateDetails.xml file.
• Paste the 1.5 positions section over the top of the old positions section. You can completely eliminate the old postions.

We are now going to repeat that task with another section of the templateDetails.xml file. In a complicated template there will be a section of the templateDetails.xml file called params that contains numerous settings for the template. In a simple template there will be very few or even none.

In Joomla 1.5 templates, the code for this section was written like this:

Now the code is a bit different, so you can't just copy this section and put it in the new xml file. The format for this section is now substantially different:

Everything that was once a "param" is now a field inside a fieldset. The parameters for simple templates are quick to move from 1.5 to 2.5, but modifying complicated templates can be very time consumiing if you have to type everything.

Be sure to type in the opening tags correctly as in the image below:

And be sure to end with these closing tags.

• Go inside the /templates/atomicVersatility/ folder and open your new index.php file.
• Delete everything between and including the opening and closing body tags.
• Go inside the old Joomla 1.5 /templates/yourtemplatename/ folder and open the index.php file.
• Copy everything between and including the opening and closing body tags onto your clipboard.
• Go inside the /templates/atomicVersatility/ folder and open your new index.php file.
• Paste everything on your clipboard between the opening and closing body tags in atomicVersatility/index.php.

• Go inside the old Joomla 1.5 /templates/yourtemplatename/ folder and open the index.php file.
• Look for these basepath lines or anything similar. Copy them to your clipboard.

define( 'YOURBASEPATH', dirname(__FILE__) );
require( YOURBASEPATH.DS."rt_styleswitcher.php");

• Go inside the /templates/atomicVersatility/ folder and open your new index.php file.
• Paste in the new basepath lines directly beneath the line "defined ("_JEXEC") or die". It should look like the following image:

• Go inside the old Joomla 1.5 /templates/yourtemplatename/ folder and open the index.php file.
• Look for JHtml::_('behavior.framework', true): paste your code there.

• Under $app =Jfactory::getapplication(); there will be line for $live_site. Change $mainframe to $app. For example, find the following line:

$live_site = $mainframe->getCfg('live_site');

• Change those lines so that it now reads like this:

$live_site = $app->getCfg('live_site');

• The entire section should end with the closing ?> tag , and come before the DOCTYPE.

• Go inside the old Joomla 1.5 /templates/yourtemplatename/ folder and open the index.php file.
• Check the head section and look for additional lines of code. Copy them to your clipboard.
• Go inside the /templates/atomicVersatility/ folder and open your new index.php file.
• Paste the code into your new index.php between the head tags. Be sure to include the opening and closing PHP tags.

• In some cases your converted template may not be shown in the Template Manager even though all coding appears to be correct. Verify that your template files are installed in the /templates/ directory. Then run the Discover process as follows.
• Go to Administrator > Extensions > Extension Manager > Discover.
• Click the Discover icon.
• If your template appears on the list, select it and then click the Install icon.

• Return to the Template Manager and set it as the default by clicking the star. That will complete the process of moving your template from Joomla 1.5 to 2.5.