Applying a Change to All Fields (selection)

Hi all, first post and hopefully a simple question. We have recently concluded activities for 2015 and we're now preparing to get an early start on 2016 activities. I want to organize cases by the year where we began activities. Because the calendar dates on file aren't going to be accurate, I've created a new selection field with two options: "2015" or "2016". The default is set to "2016"; all new cases will show "2016" even if they were started late in the year 2015. This is good.

However, I still have hundreds of existing cases that have no value in this field. Is it possible to apply the "2015" selection to all existing cases?