Hi,
long datatypes cause chaining in Oracle.
LOB is better because it can use different storage than long data type (which is stored in table row).

In order to convert:
1) Leave the long column in the old table and add a new LOB column using the ALTER TABLE command. The ALTER TABLE command is not able to change the type of a LONG column to a LOB column. LONG and LOB columns are two distinct datatypes, so it is not possible to assign a LONG column to a LOB column.
2) Write the data in the LONG or LONG RAW to a flat file
3) Use DBMS_LOB.LOADFROMFILE() procedure

cheers,
R.