You can use the start-domain command to upgrade Application Server 8.x or 9.0 domains to Application Server 9.1. Use one of the following ways to upgrade your domain:
Perform an in-place upgrade of the Application Server binaries.
When you run start-domain on the domains pointing to the earlier version of Application Server, asadmin invokes the asupgrade command , and the domains are automatically upgraded in-place.
Perform a side-by-side upgrade of the Application Server binaries.
Run start-domain on the domains of your earlier installation. The asupgrade command upgrades the domains to the domains root of the latest Application Server installation. In this scenario, the target directory for the upgrade is defined in the AS_DEF_DOMAINS_PATH in the asenv.conf.