If u want to change from no archive to archive log no data will be lost.for details go thru. this following documentation.
To switch a database's archiving mode between NOARCHIVELOG and ARCHIVELOG mode, use the SQL statement ALTER DATABASE with the ARCHIVELOG or NOARCHIVELOG option. The following statement switches the database's archiving mode from NOARCHIVELOG to ARCHIVELOG:
ALTER DATABASE ARCHIVELOG;
Before switching the database's archiving mode, perform the following operations:
Shut down the database instance.
An open database must be closed and dismounted and any associated instances shut down before you can switch the database's archiving mode. You cannot disable archiving if any datafiles need media recovery.
Back up the database.
Before making any major change to a database, always back up the database to protect against any problems.
Start a new instance and mount but do not open the database.
To enable or disable archiving, the database must be mounted but not open.
Switch the database's archiving mode.
After using the ALTER DATABASE command to switch a database's archiving mode, open the database for normal operation. If you switched to ARCHIVELOG mode, you should also set the archiving options--decide whether to enable Oracle to archive groups of online redo log files automatically as they fill.
For enabling automatic archive mode channge it in parameter file.
In order to change the database mode from ARCHIVELOG to NONARCHIVELOG mode perform the following steps:
1) Shutdown the database
2)Take a cold database backup of all the
3) In init.ora parameter file add the following parameters