When migrating security, there are really two scenarios proposed to artifacts on your target environment: clean or dirty. If you have already done a clean migration (that is, the first time) to your target once, then the environment is now dirty. So, subsequent migrations must take into consideration the possibility of overwriting or replacing existing items. Regarding Security Realm migration, the WLS Security Realm migration utility handles all of this for you. For example, if you export a Security Realm from Development and it contains a group named BIAdministrators that also exists in the Production environment, the import into Production will not overwrite the BIAdministrators group already existing there.
To understand how to migrate the default Security Realm from one environment to another, follow these steps:
Open the WLS Administration Console and navigate to Security Realms > myrealm.
Click the Migration tab.
Click the Export sub-tab...