The Symfony blog has posted a quick guide with a tip to upgrading to Twig 2.0 made simpler thanks to some built-in deprecation notices.

As Twig 2.0 is approaching fast now, it's time to focus on how to boost adoption of this new major version. [...] Also, the good news is that most of the time, it's possible to make your code work for both major versions (that's important for shared code like bundles). [...] How to make it easy? Like for Symfony, I've now added deprecation notices throughout the code to give Twig users an easy way to upgrade their code (this is available in the upcoming Twig 1.21 version).

The release of v1.21 comes with the deprecation notices already built-in making it easy to find the issues before making the jump up to v2.0. He also includes mentions of two other things included to make it easier: a deprecation collector to gather the issues in one place and a visitor base class diff that can make them compatible with both versions (1.21 and pre-1.21).