I have a project (developed by others) running on Symfony 2.0.15 and I need to upgrade it to newest Symfony2 version cause I need to add some new code and it's been hard to get it done. I can't find enough information for this, can any give me some links or experiences around migration process? Will be fine to migrate directly from that version to 2.6.6 (stable) or 2.7 (beta)? What would you do and how?
You can add version 2.7 replace with 2.0.15 into composer.json. Then run the command php composer.phar install. then your project will be upgraded to 2.7.
This link will be helpful for you http://symfony.com/doc/current/cookbook/upgrading.html