There's no "trivial" way to turn a partitioned table into a composite partitioned table in 12.2; here's an article that demonstrates the sort of thing you'd have to do: https://jonathanlewis.wordpress.com/2019/05/23/re-partitioning/
If you were running 18c or 19c then the the "alter table X modify partition" syntax would work - if you had the time and resources to let it run (and allow for a bug in 18c shown in this note), e.g.
alter table t_already_there modify
PARTITION BY RANGE (start_date) INTERVAL(NUMTOYMINTERVAL(1, 'MONTH'))
SUBPARTITION BY LIST (keep_flag)
( SUBPARTITION p_t VALUES ('Y')
, SUBPARTITION to_others values (default)
PARTITION P1_DATE1 VALUES LESS THAN (TO_DATE(' 2020-06-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIAN'))
For an example of dealing with a messier setup (e.g. a partitioned table with a history of tweaks and changes) there's a follow-up to the first example.
your link showed me the way, I thought Id seen every new feature in 12.2 but create table for exchange passed us by. Thats a gamechanger for us and will use it here and many other places. Thanking you as always!!!!