Oracle: change DB to archivelog mode

1. Start a new instance and mount, but do not open, the database.
   

   STARTUP MOUNT
   

   To enable or disable archiving, the database must be mounted but not open.
   
2. Change the database archiving mode. Then open the database for normal operations.
   

   ALTER DATABASE ARCHIVELOG;
   ALTER DATABASE OPEN;
   

3. Shut down the database.
   

   SHUTDOWN IMMEDIATE
   

4. Back up the database.
   Changing the database archiving mode updates the control file. After changing the database archiving mode, you must back up all of your database files and control file. Any previous backup is no longer usable because it was taken in NOARCHIVELOG mode.