Symfony2 LTS: how to upgrade from 2.3 to 2.7?

Symfony 2.7 was released on 30th April 2015 and is the current LTS (Long Term Support) version after the 2.3 version. Maintenance for these versions will end on May 2016 for Symfony 2.3 and May 2018 for Symfony 2.7. Security fixes will be released during one year after end of maintenance for both versions.

As suggested by Massimiliano Arione in the announce comments, what are the changes required to upgrade from Symfony 2.3 from 2.7 without having to check all the minor upgrades (2.3 → 2.4, 2.4 → 2.5, etc.)?

As reminded by Med in a comment, Symfony2 developers have tried to keep backward compatibility in the 2.x branch. So as long as you don't want to switch to the 3.0 branch later, you can ignore the changes between 2.3 and 2.7 because they are mostly deprecation changes.

In order to upgrade you app from Symfony 2.3 to Symfony 2.7 you'll have to update your composer.json file:

([…] indicates unchanged code)

Old (2.3) version:

{
    […]
    "autoload": {
        "psr-0": { "": "src/" }
    },
    "require": {
        "php": ">=5.3.3",
        "symfony/symfony": "2.3.*",
        "doctrine/orm": "~2.2,>=2.2.3,<2.5",
        "doctrine/dbal": "<2.5",
        "doctrine/doctrine-bundle": "~1.2",
        "twig/extensions": "1.0.*",
        "symfony/assetic-bundle": "~2.3",
        "symfony/swiftmailer-bundle": "~2.3",
        "symfony/monolog-bundle": "~2.4",
        "sensio/distribution-bundle": "~2.3",
        "sensio/framework-extra-bundle": "~3.0,>=3.0.2",
        "sensio/generator-bundle": "~2.3",
        "incenteev/composer-parameter-handler": "~2.0"
    },
    "scripts": {
        "post-install-cmd": [
            "Incenteev\\ParameterHandler\\ScriptHandler::buildParameters",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::prepareDeploymentTarget"
        ],
        "post-update-cmd": [
            "Incenteev\\ParameterHandler\\ScriptHandler::buildParameters",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::prepareDeploymentTarget"
        ]
    },
    […]
    "minimum-stability": "stable",
    "extra": {
        […]
        "incenteev-parameters": {
            "file": "app/config/parameters.yml"
        },
        "branch-alias": {
            "dev-master": "2.3-dev"
        }
    }
}

New (2.7) version:

{
    […]
    "autoload": {
        "psr-4": { "": "src/", "SymfonyStandard\\": "app/SymfonyStandard/" }
    },
    "require": {
        "php": ">=5.3.9",
        "symfony/symfony": "2.7.*",
        "doctrine/orm": "^2.4.8",
        "doctrine/doctrine-bundle": "~1.4",
        "symfony/assetic-bundle": "~2.3",
        "symfony/swiftmailer-bundle": "~2.3",
        "symfony/monolog-bundle": "~2.4",
        "sensio/distribution-bundle": "~4.0",
        "sensio/framework-extra-bundle": "^3.0.2",
        "incenteev/composer-parameter-handler": "~2.0"
    },
    "require-dev": {
        "sensio/generator-bundle": "~2.3",
        "symfony/phpunit-bridge": "~2.7"
    },
    "scripts": {
        "post-install-cmd": [
            "Incenteev\\ParameterHandler\\ScriptHandler::buildParameters",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::removeSymfonyStandardFiles",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::prepareDeploymentTarget"
        ],
        "post-update-cmd": [
            "Incenteev\\ParameterHandler\\ScriptHandler::buildParameters",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::removeSymfonyStandardFiles",
            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::prepareDeploymentTarget"
        ]
    },
    […]
    "extra": {
        […]
        "symfony-assets-install": "relative",
        […]
        "branch-alias": {
            "dev-master": "2.7-dev"
        }
    }
}

Summary:

Symfony version is updated
PSR-4 is used instead of PSR-0
twig/extensions is not installed by default, you may need to add it if you use Twig extensions
sensio/generator-bundle is required only in dev environment
scripts part has been updated
"minimum-stability": "stable", has been removed

Once you have updated your composer.json file, you have to update the dependencies:

composer update --prefer-dist -vv

Then you may need to flush the cache:

php app/console cache:clear --env=dev

Note: I used the following command in order to get the composer.json files:

# create Symfony "2.3.*" project in the "2.3" directory
composer create-project symfony/framework-standard-edition "2.3" "2.3.*" --no-interaction -v
# create Symfony "2.7.*" project in the "2.7" directory
composer create-project symfony/framework-standard-edition "2.7" "2.7.*" --no-interaction -v
# compare the Symfony 2.3 and 2.7 composer.json files
diff -u 2.3/composer.json 2.7/composer.json

(we use 2.3.* and not 2.3 because we want the last version (2.3.31 today) and not the initial release (2.3.0))

The diff is available at GitHub too, but the composer.json file of Symfony 2.3 has been updated multiple times, so it may be different from your file.