I am a bit concerned about https. There will be definitely problems with hotlinked images, they will generate tons of warnings, as they will not use the same certificate and they are not always hosted on servers with https. At other forums I know it ended with hosting a copy of every image on the forum server (costly in terms of space and traffic).
The upgrade itself won't be visible, latest changes (since at least 2.0.12 where I stopped installing them) were mostly related to security of some obscure functions that are not enabled at CF.