You need to be careful there. Migration and upgrading are 2 different things. Migration is generally referred to when an application running on weblogic 8.1 is deployed on weblogic 9.2 Whereas, upgradation means you are upgrading your installation of weblogic 8.1 to weblogic 9.2. Not sure but I do not think there is any migration guide provided by weblogic on this. Depending on what your application is, what components it consists of you would have different steps to migrate. A good starting point is changing the jdk version used for building the code. weblogic 9.2 ships with jdk 1.5 as default.
He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever - Chinese proverb