@Entity
@org.hibernate.annotations.Entity( dynamicInsert=true, dynamicUpdate=true )
@Table(name="GUARD",
      uniqueConstraints={@UniqueConstraint(columnNames={"name"})})
public class Guard implements Serializable {
    ...
    @OneToMany(mappedBy="guard", cascade={CascadeType.ALL}, fetch = FetchType.LAZY )
    private List<Device> devices = new ArrayList<Device>() ;
    ...
}

@Entity
@org.hibernate.annotations.Entity( dynamicInsert=true, dynamicUpdate=true )
@Table(name="DEVICE",
      uniqueConstraints={@UniqueConstraint(columnNames={"identifier"})})
public class Device implements Serializable {
    ...
    @ManyToOne(fetch = FetchType.EAGER )
    @JoinColumn(name="GUARD_FK_ID")
    @org.hibernate.annotations.ForeignKey(name = "GUARD_FK")
    private Guard guard ;
    ...
}

   @Transactional(propagation = Propagation.REQUIRED, isolation = Isolation.DEFAULT)
   public void reassignDevicesTo(Guard from, Guard to) {
      Guard fromAttached =  entityManager.getReference( Guard.class, from.getId() ) ;
      List<Device> fromDevices = fromAttached.getDevices() ;
      Guard toAttached =  entityManager.getReference( Guard.class, to.getId() ) ;
      List<Device> toDevices = toAttached.getDevices() ;
      //reassign
      toDevices.addAll( fromDevices ) ;
      fromDevices.clear() ;
   }