Now I have the experience of a completed, successful, content upgrade. We're on 11.8!
We did it with the help of the Apptio upgrade team. It took us three weeks of freeze time, and another three weeks of prep time before that.
The outcome was satisfactory since we completed the upgrade without interruption of the service to our end users, all pre-existing use cases continued to work, and we got new content that we're now in the process of adjusting and enabling.
My only recommendation is to have close dialog between the lead TBMA and the upgrade team. The more the better. I'm not saying we were lacking communication, just that I whish we had even more, and go more into the hidden technical detail of the new content.