SQL> create table t (c1 clob, c2 long);

Table created.

SQL> alter table t modify (c1 number, c2 number);
alter table t modify (c1 number, c2 number)
                      *
ERROR at line 1:
ORA-22859: invalid modification of columns

SQL>select banner from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - 64bit Production
PL/SQL Release 11.1.0.6.0 - Production
CORE    11.1.0.6.0      Production
TNS for 64-bit Windows: Version 11.1.0.6.0 - Production
NLSRTL Version 11.1.0.6.0 - Production

5 rows selected.

SQL>create table t12
  2  (col1 long);

Table created.

SQL>insert into t12 values('1234');

1 row created.

SQL>alter table t12 modify col1 number;
alter table t12 modify col1 number
                       *
ERROR at line 1:
ORA-01439: column to be modified must be empty to change datatype