Is it possible to send ONLY digest emails?

I know we can trigger to not have digests, but is the opposite possible?

Or would I have to change the preferences of every user?

You can change the user defaults in your site settings, but you must do it before you have any users, as the defaults are inherited at the time of user signup.

You could also run a Ruby/Rails command line to force the preferences for all existing users, but nothing stops users from changing their preferences back to the way they want it, either.