当前状态是dg1为主库，备库为dg2
先检查主备库状态：
dg1

SQL> select name,database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
DATABASE_ROLE    OPEN_MODE  SWITCHOVER_STATUS
---------------- ---------- --------------------
PRIMARY          READ WRITE SESSIONS ACTIVE

dg2

SQL> select database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
DATABASE_ROLE    OPEN_MODE  SWITCHOVER_STATUS
---------------- ---------- --------------------
PHYSICAL STANDBY MOUNTED    NOT ALLOWED

现在dg1（当前主库）上操作以下步骤： 

SQL>  alter database commit to switchover to physical standby with session shutdown;
Database altered.
SQL> shutdown immediate;
ORA-01507: database not mounted
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area  390070272 bytes
Fixed Size                  2021024 bytes
Variable Size             150997344 bytes
Database Buffers          234881024 bytes
Redo Buffers                2170880 bytes
Database mounted.
SQL> select switchover_status from v$database;
SWITCHOVER_STATUS
--------------------
TO PRIMARY

现在dg2（当前备库）上操作以下步骤： 

SQL> select name,database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
NAME      DATABASE_ROLE    OPEN_MODE  SWITCHOVER_STATUS
--------- ---------------- ---------- --------------------
ORCLDB    PHYSICAL STANDBY MOUNTED    TO PRIMARY
SQL> select switchover_status from v$database;
SWITCHOVER_STATUS
--------------------
TO PRIMARY
SQL>  alter database commit to switchover to primary;
Database altered.
SQL> alter database open;
Database altered.
SQL> select name,database_role,OPEN_MODE,SWITCHOVER_STATUS from v$database;
NAME      DATABASE_ROLE    OPEN_MODE  SWITCHOVER_STATUS
--------- ---------------- ---------- --------------------
ORCLDB    PRIMARY          READ WRITE SESSIONS ACTIVE


以上的操作已经完成了主备的切换，现在dg2为主库，dg1为备库，接下来我们在现在的主库dg2上做几次日志变化，看dg1能否同步过来。
dg1上看已经归档的日志：

SQL> select registrar,creator thread#,sequence#,first_change#,next_change# from v$archived_log;
REGISTR THREAD#  SEQUENCE# FIRST_CHANGE# NEXT_CHANGE#
------- ------- ---------- ------------- ------------
FGRD    FGRD            29        586174       586196
dg2上经过几次日志切换，
SQL> select * from v$log;
GROUP#    THREAD#  SEQUENCE#      BYTES    MEMBERS ARC STATUS           FIRST_CHANGE# FIRST_TIM
---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- ---------
1          1         36   52428800          1 NO  CURRENT                 586469 05-JAN-14
2          1         34   52428800          1 YES INACTIVE                586464 05-JAN-14
3          1         35   52428800          1 YES INACTIVE                586467 05-JAN-14

可以看出现在日志已经到了36，那么我们在dg1上应用日志，应该会应用到日志35；
以下操作验证：

SQL> alter database recover managed standby database disconnect from session;
Database altered.
SQL> select registrar,creator thread#,sequence#,first_change#,next_change# from v$archived_log;
REGISTR THREAD#  SEQUENCE# FIRST_CHANGE# NEXT_CHANGE#
------- ------- ---------- ------------- ------------
RFS     ARCH            34        586464       586467
RFS     ARCH            35        586467       586469

以上的主备切换过程就已经顺利进行完成。