In my yang model I have enum defined as follows :

typedef deployment-mode {
type enumeration {
enum REACTIVE_MODE

{ description "Default deployment mode. "; }

enum PROACTIVE_MODE

{ description "Hybrid deployment mode."; }

enum INTEGRATED_MODE

{ description "Integrated mode."; }

}
}

generated sources, generated using org.opendaylight.yangtools.maven.sal.api.gen.plugin.CodeGeneratorImpl contains values for this enumeration without underscore - REACTIVEMODE,PROACTIVEMODE,INTEGRATEDMODE. It would be nice to have way, how to preserve underscore in enum values.