There are a number of migration resources in the Struts 2 Guides Struts 2 is not backwards-compatible, however it is possible to have 1.x and 2 run alongside each other:
Add the Struts 2.0 JARs to your exisitng Struts 1.3 application. Configure S2 to handle *.action requests and let S1 handle *.do requests. S1 and S2 can run side by side, within the same web application. Teams can implement new features in the latest version, and migrate older pieces as needed. Optionally, some common resources may be shared between S1 and S2, like messages, validation rules, and Tiles configurations, making for an even smoother migration.