Unfortunately, this isn’t an update. It’s a new app (iOS & Mac). It will be released February 4th with a 50% discount the first week. The original Day One app will continue to be supported.
New features include multiple journals and multiple photos in a single journal entry. I look forward to tagging multiple entries at a time. Some of the standard features from the original Day One (publishing for one) won’t be immediately available in DO2 but will be added in future updates. IFTTT support and book printing to a printing service are in the works. There is also an Android version planned.