While doing the partition key update and if the value execeeds the partition limit, the update failes with the error

ORA-14402: updating partition key column would cause a partition change

Refering to the FAQ
http://www.dbasupport.com/oracle/faq/Detailed/8.shtml update on partition key is not possible outside the partition limits.

But in oracle 8i onwards it is possible to update the partition keys directly with out delete and inserte.

Alter the partition table or create the table with the option "Enable Row Movement"

Alter table Table_name enable row movement;