At the moment for every Database upgrade a downtime is needed. a startup RESTRICT or MIGRATE is required to update objects belonging to SYS.
But Oracle introduced a feature which should overcome this limitation:
With Edition Based Redefinition 2 different versions of Views and Packages can exist at the same time supporting different setups during a migration.
In a RAC environment, the new edition could be loaded before the first instance is moved from the old to the new version. Then every instance with higher binaries uses the new edition, whereas the old instances are using the old views and packages.
After the last instance is upgraded, a post-script makes the new edition default and removes the old one.
It's not easy, but HA rarely is.