How to upgrad to a major version of symfony with composer : I've an old symfony project and here is composer.json :

    {
    "name": "symfony/framework-standard-edition",
    "license": "MIT",
    "type": "project",
    "description": "The \"Symfony Standard Edition\" distribution",
    "autoload": {
        "psr-0": {"": "src/"}
    },
    "require": {
        "php": ">=5.5.9",
        "symfony/symfony": "*",
        "symfony/console": ">=2.2.3",
        "doctrine/orm": "~2.2,>=2.2.3",
        "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",
        "sensio/generator-bundle": "~2.3",
        "incenteev/composer-parameter-handler": "~2.0",
        "facebook/php-sdk-v4" : "4.0.*",
        "symfony/finder": "^3.1",
        "sonata-project/core-bundle": "^3.1",
        "sonata-project/seo-bundle": "^2.0",
        "sonata-project/admin-bundle": "^3.6"
    },
    "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"
        ],
        "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"
        ]
    },
    "config": {
        "bin-dir": "bin"
    },
    "extra": {
        "symfony-app-dir": "app",
        "symfony-web-dir": "web",
        "incenteev-parameters": {
            "file": "app/config/parameters.yml"
        },
        "branch-alias": {
            "dev-master": "2.4-dev"
        }
    } }

First I'd use : php composer.phar self-update 
Then I used below command to upgrade :
php composer.phar update
And here is the result :

>  Search for a package: symfony
> 
> Found 15 packages matching symfony
> 
>    [0] symfony/symfony    [1] rgies/symfony    [2] php-tmdb/symfony   
> [3] webforge/symfony    [4] lexpress/symfony1    [5] symfony/symfony1 
> [6] sabaki-dev/symfony1    [7] behat/symfony2-extension    [8]
> escapestudios/symfony2-coding-standard    [9] smarkio/symfony1   [10]
> languara/symfony3   [11] m6web/symfony2-coding-standard   [12]
> irongit/symfony2-stream-response   [13]
> polishsymfonycommunity/symfony2-mocker-extension   [14]
> symfony/console
> 
> Enter package # to add, or the complete package name if it is not
> listed: 0 Enter the version constraint to require (or leave blank to
> use the latest version): Using version ^3.1 for symfony/symfony Search
> for a package: ./composer.json has been updated Loading composer
> repositories with package information Updating dependencies (including
> require-dev) Your requirements could not be resolved to an installable
> set of packages.
> 
>   Problem 1
>     - Conclusion: don't install symfony/symfony v3.1.6
>     - Conclusion: don't install symfony/symfony v3.1.5
>     - Conclusion: don't install symfony/symfony v3.1.4
>     - Conclusion: don't install symfony/symfony v3.1.3
>     - Installation request for sensio/generator-bundle (locked at v2.5.3, required as ~2.3) -> satisfiable by
> sensio/generator-bundle[v2.5.3].
>     - Conclusion: don't install symfony/symfony v3.1.2
>     - Conclusion: don't install symfony/symfony v3.1.1
>     - don't install symfony/finder v3.1.6|don't install symfony/symfony v3.1.0
>     - don't install symfony/symfony v3.1.0|remove symfony/finder v3.1.6
>     - don't install symfony/symfony v3.1.0|don't install symfony/finder v3.1.6
>     - Installation request for symfony/symfony ^3.1 -> satisfiable by symfony/symfony[v3.1.0, v3.1.1, v3.1.2, v3.1.3, v3.1.4, v3.1.5,
> v3.1.6].
>     - Installation request for symfony/finder (locked at v3.1.6, required as ^3.1) -> satisfiable by symfony/f inder[v3.1.6],
> symfony/symfony[v3.1.6].
> 
> 
> Installation failed, reverting ./composer.json to its original
> content.


