That's one great thing about child themes I suppose - it makes theme updates less painful.

I was wondering though: is WP going to introduce a Changelog system for themes as well - like they have done with plugins (as championed by Jeff way back when?) because not every theme author maintains a separate website with a changelog.

Not knowing if this is possible, but perhaps one way of ensuring that people will upgrade (without doing it automatically) is to have a 'revert' button along with the 'upgrade' button? But conditionally in that if its a security update, then no revert button is shown. But if its a more 'optional' upgrade, then it will. And this setting can be determined by theme or plugin authors?