If you are looking at a website with an older version of Joomla, you may be facing several hours of fixing the site to look acceptable and contain all the previous content. You may ask "Why bother?"

Support

Joomla.org has ending support for version 2.5 or older. Official support for Joomla 2.5 ended on December 31, 2014.

This means that there won't be any more bug fixes or security upgrades. Along with the Joomla installation itself, any template or extension for older versions of Joomla that you have installed will no longer be supported or upgraded either.

Your Joomla 1.5 or Joomla 2.5 site will still function, but there won't be any more upgrades issued by Joomla and any security issues that remain will leave an open door for hackers.

Appearance

The modern templates reflect the way websites look now, shiny, responsive and mobile ready. New templates won't work with the old releases. Any visitor can spot a old tired looking layout. It doesn't reflect well on the company or product being represented. 

Functionality

There are many new extensions for Joomla for modern functionality, such as extensions for social sites, e-commerce, forums and to be mobile ready. If you have an older version of Joomla, you can not use these new extensions.

Responsiveness

With the variety of devices that use the internet, your website needs to look good and function well on everything from 55" televisions to tablets to laptops to smart phones. The new releases of Joomla have responsive built in and the newer templates use this to make your site look good on any device.

PHP upgrades

Your hosting environment may at some point no longer offer the older versions of PHP that are required to run Joomla 1.5 and Joomla 1.5 will not work with the latest releases of PHP.

Security

On top of the appearance, responsiveness and functionality, you also need to consider the security of the website. Older sites have vulnerabilities that allow hackers to bring down the site or steal data contained in the site. Once you migrate the Joomla site to 3.+, it is easier to keep up with the minor upgrades with their security patches.

SEO, Search Engine Optimization

The newer Joomla releases and available extensions can make your site friendlier to the search engines web crawlers.