I have migrated about 40 websites from Joomla 2.5 to Joomla 3.x. This article contains my notes, including gotcha's, along with documentation.

Preparing for a Joomla 2.5 Migration

Preperation is key to success.

Backup Your Joomla 2.5 Installation

First make a backup of your existing website. This could be with Akeeba Backup or using your Server's GUI control panel or FTP, or tarball, but I prefer and recommend using Git.

If possible make sure your backup works! Having a reliable restore point can save you hours of work (again easy with Git).

Preparing Your Server

Next you want to ensure that your server supports PHP 5.4 + and MySQL 5.1 + (InnoDB support required) and all other Joomla requirements

Peparing Joomla

Make sure you have updated to the lastest version of Joomla (2.5.28).

Make sure you have the latest database schema by going to: Extension Manager > Database tab (click fix if it's not).

Extensions

Go to the Extention manager, in the manage section. Remove any extentions you are not using. Make a list of all the third party extentions you plan on migrating. Check to see if the extentions exist for Joomla 3.0, if they don't you will have to find alternatives.

The Update From Joomla 2.5 to Joomla 3.x

  1. Go to: System > Global Configuration > Server tab. Set Error Reporting to Maximum
  2. Go to: Extensions > Plugin Manager > Remember Me. Disable the plugin.
  3. Go to: Components > Joomla Update
  4. Click: Options. Change Update server to Short Term Support, Save & close
  5. Note: If the update is not showing up, go to Extension manager > Update and press Purge Cache from the toolbar.
  6. Run the update

When the updates are complete you can do the following:

  1. Purge Cache
  2. Enable Remember Me
  3. Enable Cache