I'm not sure if my answer is relevant. You might've upgraded yet. Anyway, I'm answering just not ot break the chain ).
Oracle told me that my approached would worked and it did!
I followed this: http://ora00600.com/articles/data-guard-physical-standby-upgrade.html. I created my own steps according to my enviroment and the whole process went smoothly.
Just one advice - go through all steps on your test environment to eliminate number of unexpected errors during you production upgrade as you probably won't have time to solve them when they appear.
All the best,