Use alter table ... move partition to move a partition (and its index) to a specified segment.
alter table table_name move partition partition_name to destination_segment_name
partition_name – the partition you are moving.
destination_segment_name – a new or existing segment to which you are moving the partition. You cannot specify “default” as the destination_segment_name.
See the Reference Manual: Commands.
You must enable select into/bulkcopy to issue alter table ... move partition.