In all the IBM documentation, they say to shut down the DB2 license and all instances prior to the install. After the install, you migrate the DAS and instances. Why do you have to shut down the license and instances prior to the install? AIX is always a "new" install, not an upgrade. We install into a different directory than the prior version. Could the install and fix packs be done several days, and while a server is running, before the actual migration?
You can install a new version of DB2 (ie. v9) ahead of time and while an existing version (ie. v8) is up and running. When you're ready to migrate your existing instance/database, this is when you would need to stop all processes associated with the instance/database you're migrating.