Switching Player Side

Once a game has started and player sides have been assigned, is there a way to swap sides? I have a player that is trying to join, but can only view as an observer.

When loading the save game file, returning players are automatically loaded as their side, but new people are only given the observer option.

At least one player must retire from the side they occupy and become an observer before the game is saved (or a logfile being recorded is ended). Only then will there be available sides to join.