Did you enjoy migrations with Symfony 1.x versions ? Do you have some additional months to waste ? Rewrite your apps for Symfony 2.0 !
Just look at the preview of what you will have to relearn, and how you will have to rewrite your code. That’s the price to pay, if you dare think about migrating your applications to benefit from Symfony 2.0 improvements (like performance). Indeed, backward compatibility seems to be the last matter Symfony core team pays attention to.
Did PHP developers kill BC from version 4 to 5 ?…
You’ve been warned : Symfony is a framework for professionals… in code rewriting.