Stefan Koopmanschap has written up an article on the Leaseweblabs.com blog about migrating a Symfony 1 application to Symfony2 in a (slightly) less painful way that making the move all at once.
It is much easier to do a gradual migration. Start with one part of your application, and bit by bit migrate your logic and application. The traditional way of doing such migrations is to create a new project and have parallel development on the old and the new version of the application. The problem with this, though, is that when you make a change to your old application, you have to make the same change in the new codebase, essentially doubling the amount of work for each feature you need to implement. [...] You could wrap your old application into your Symfony2 application, and have different parts of your application be handled by different versions of your codebase.
With the help of a bundle he created, IngewikkeldWrapperBundle that handles the rerouting of your requests based on where the requested resource exists (in the Symfony 1 or 2 codebase).