Voilà à peu près mon point de départ

refactor_start.rb

def change_password
  @user = User.find(params[:id])
  authorize! :update, @user
  @user_input = params.require(:user)

  if (@user.valid_password?(@user_input['old_password']))
    if (@user_input['password'] == @user_input['password_confirmation'])
      @user.password = @user_input['password']
      save_successful = @user.save

      if save_successful
        redirect_to root_path, :notice => 'Mot de passe mis à jour'
      else
        flash[:error] = t('activerecord.errors.models.user.attributes.password.too_short')
        render :password
      end
    else
      flash[:error] = t('messages.users.password_dont_match')
      render :password
    end
  else
    flash[:error] = t('messages.users.invalid_old_password')
    render :password
  end

end