After once having selected the version, that should be shown in the editor (right column) I can't change to the other version any more. I would regard it as helpful if a button would be added that makes such a change possible. The button should create a message that informs the user that if the version is changed now any changes made in the right column will be overwritten by the version to which is changed, followed by the buttons "change to ...version" and "cancel".
The feature is very helpful and improves the solving of edit conflict a lot.