Upgrade information
Read the upgrade guides in symfony/symfony github repository:
There are also changelogs in the repository:
Backup your project!
Upgrading with composer
Get composer and put the symfony 2.3 standard edition's composer.json into your project root.
Now run composer update
with your project's patch as working directory.
-> You will face some exceptions due to breaking changes exceptions. Google will help resolve those :)
Personal experience
In my legacy projects the most noticeable bc break from 2.0 to 2.1 was this one related to the form-builder because I had to update every form.
Before ( 2.0 )
use Symfony\Component\Form\FormBuilder;
public function buildForm(FormBuilder $builder, array $options)
After ( 2.1+ )
use Symfony\Component\Form\FormBuilderInterface;
public function buildForm(FormBuilderInterface $builder, array $options)