There is a major change on layer 2 configuration in this upgrade from the old ethernet switching model to the new ELS (enhanced layer 2 services) method.
You can run your existing configuration through the conversion tool.
https://www.juniper.net/customers/support/configtools/elstranslator/index.jsp
And see the overview of the ELS system here.
http://www.juniper.net/techpubs/en_US/junos/topics/task/configuration/getting-started-els.html