Non-Partition table to Partition

Folks ,
I have a table with about a 2 mil records. Now we want to make this tables partitioned by range on a different tablespace. Since this table being used we would like to do it in a easier way with very minimal downtime (other than exp/imp).Mind you this table has Foreign key and Pk constraints, Please help me out with the startegies/steps involved and drawbacks.
Thanks

create new table - insert append into new table then rename it and create the constraints

DBMS_REDEFINITION package.

No downtime, you will be actualy doing it online, that is while transactions are going on on that table.