Modify an existing DBInstance.

        :type id: str
        :param id: Unique identifier for the new instance.

        :type security_groups: list of str or list of DBSecurityGroup objects
        :param security_groups: List of names of DBSecurityGroup to authorize on
                                this DBInstance.

        :type preferred_maintenance_window: str
        :param preferred_maintenance_window: The weekly time range (in UTC)
                                             during which maintenance can
                                             occur.
                                             Default is Sun:05:00-Sun:09:00

        :type master_password: str
        :param master_password: Password of master user for the DBInstance.
                                Must be 4-15 alphanumeric characters.

        :type allocated_storage: int
        :param allocated_storage: The new allocated storage size, in GBs.
                                  Valid values are [5-1024]

        :type instance_class: str
        :param instance_class: The compute and memory capacity of the
                               DBInstance.  Changes will be applied at
                               next maintenance window unless
                               apply_immediately is True.

                               Valid values are:

                               * db.m1.small
                               * db.m1.large
                               * db.m1.xlarge
                               * db.m2.xlarge
                               * db.m2.2xlarge
                               * db.m2.4xlarge

        :type apply_immediately: bool
        :param apply_immediately: If true, the modifications will be applied
                                  as soon as possible rather than waiting for
                                  the next preferred maintenance window.

        :type backup_retention_period: int
        :param backup_retention_period: The number of days for which automated
                                        backups are retained.  Setting this to
                                        zero disables automated backups.

        :type preferred_backup_window: str
        :param preferred_backup_window: The daily time range during which
                                        automated backups are created (if
                                        enabled).  Must be in h24:mi-hh24:mi
                                        format (UTC).

        :type multi_az: bool
        :param multi_az: If True, specifies the DB Instance will be
                         deployed in multiple availability zones.

        :rtype: :class:`boto.rds.dbinstance.DBInstance`
        :return: The modified db instance.

def modify_dbinstance(self, id, param_group=None, security_groups=None,
                          preferred_maintenance_window=None,
                          master_password=None, allocated_storage=None,
                          instance_class=None,
                          backup_retention_period=None,
                          preferred_backup_window=None,
                          multi_az=False,
                          apply_immediately=False):
        """
        Modify an existing DBInstance.

        :type id: str
        :param id: Unique identifier for the new instance.

        :type security_groups: list of str or list of DBSecurityGroup objects
        :param security_groups: List of names of DBSecurityGroup to authorize on
                                this DBInstance.

        :type preferred_maintenance_window: str
        :param preferred_maintenance_window: The weekly time range (in UTC)
                                             during which maintenance can
                                             occur.
                                             Default is Sun:05:00-Sun:09:00

        :type master_password: str
        :param master_password: Password of master user for the DBInstance.
                                Must be 4-15 alphanumeric characters.

        :type allocated_storage: int
        :param allocated_storage: The new allocated storage size, in GBs.
                                  Valid values are [5-1024]

        :type instance_class: str
        :param instance_class: The compute and memory capacity of the
                               DBInstance.  Changes will be applied at
                               next maintenance window unless
                               apply_immediately is True.

                               Valid values are:

                               * db.m1.small
                               * db.m1.large
                               * db.m1.xlarge
                               * db.m2.xlarge
                               * db.m2.2xlarge
                               * db.m2.4xlarge

        :type apply_immediately: bool
        :param apply_immediately: If true, the modifications will be applied
                                  as soon as possible rather than waiting for
                                  the next preferred maintenance window.

        :type backup_retention_period: int
        :param backup_retention_period: The number of days for which automated
                                        backups are retained.  Setting this to
                                        zero disables automated backups.

        :type preferred_backup_window: str
        :param preferred_backup_window: The daily time range during which
                                        automated backups are created (if
                                        enabled).  Must be in h24:mi-hh24:mi
                                        format (UTC).

        :type multi_az: bool
        :param multi_az: If True, specifies the DB Instance will be
                         deployed in multiple availability zones.

        :rtype: :class:`boto.rds.dbinstance.DBInstance`
        :return: The modified db instance.
        """
        params = {'DBInstanceIdentifier' : id}
        if param_group:
            params['DBParameterGroupName'] = param_group
        if security_groups:
            l = []
            for group in security_groups:
                if isinstance(group, DBSecurityGroup):
                    l.append(group.name)
                else:
                    l.append(group)
            self.build_list_params(params, l, 'DBSecurityGroups.member')
        if preferred_maintenance_window:
            params['PreferredMaintenanceWindow'] = preferred_maintenance_window
        if master_password:
            params['MasterUserPassword'] = master_password
        if allocated_storage:
            params['AllocatedStorage'] = allocated_storage
        if instance_class:
            params['DBInstanceClass'] = instance_class
        if backup_retention_period is not None:
            params['BackupRetentionPeriod'] = backup_retention_period
        if preferred_backup_window:
            params['PreferredBackupWindow'] = preferred_backup_window
        if multi_az:
            params['MultiAZ'] = 'true'
        if apply_immediately:
            params['ApplyImmediately'] = 'true'

        return self.get_object('ModifyDBInstance', params, DBInstance)