How to change Oracle database character set?

We can change the character set of an Oracle database once the database instance is created, provided that the database has no data unless the data is corrupted. If the database has data, it is better to recreate the database instance.

And below is a step by step practical example to change oracle database character set using SQL*PLUS command-line tool.

D:\app\Admin\virtual\virtual\product\12.2.0\dbhome_1\bin>sqlplus/nolog
 SQL*Plus: Release 12.2.0.1.0 Production on Sat Jan 29 17:38:34 2022
 Copyright (c) 1982, 2016, Oracle.  All rights reserved.

SQL> conn sys/orcl@orcl as sysdba
Connected.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup restrict
ORACLE instance started.
 Total System Global Area 2499805184 bytes
Fixed Size                  8749920 bytes
Variable Size             687869088 bytes
Database Buffers         1795162112 bytes
Redo Buffers                8024064 bytes
Database mounted.
Database opened.

--- Check the existing character set
SQL> select value from NLS_DATABASE_PARAMETERS where PARAMETER = 'NLS_CHARACTERSET'; 
 VALUE
--------------------------------------------------------------------------------
WE8ISO8859P1

--- Change the existing character set to AL32UTF8
SQL> ALTER DATABASE CHARACTER SET INTERNAL_USE AL32UTF8;
 Database altered.

SQL> ALTER SYSTEM DISABLE RESTRICTED SESSION;
 System altered.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.

SQL> startup
ORACLE instance started.
 Total System Global Area 2499805184 bytes
Fixed Size                  8749920 bytes
Variable Size             687869088 bytes
Database Buffers         1795162112 bytes
Redo Buffers                8024064 bytes
Database mounted.
Database opened.
SQL> select value from NLS_DATABASE_PARAMETERS where PARAMETER = 'NLS_CHARACTERSET';
 VALUE
----------------------------------------------------------------
AL32UTF8