Ce que j'ai obtenu après avoir refactoré autant que je pouvais à l'aide d'exceptions pour supprimer les if imbriqués

refactor_exceptions.rb

def change_password
  @user = get_and_authorize_user
  @user_input = params.require(:user)

  begin
    check_old_password_match
    check_new_password_confirmation_match
    @user.password = @user_input['password']
    save_user
    redirect_to root_path, :notice => 'Mot de passe mis à jour'
  rescue PasswordExeception => exeception
    error_message_and_retry exeception.message
  end
end

private

def get_and_authorize_user
  user = User.find(params[:id])
  authorize! :update, user
  return user
end

def check_old_password_match
  if not @user.valid_password?(@user_input['old_password'])
    raise PasswordExeception, 'messages.users.invalid_old_password'
  end
end

def check_new_password_confirmation_match
  if not @user_input['password'] == @user_input['password_confirmation']
    raise PasswordExeception, 'messages.users.password_dont_match'
  end
end

def save_user
  user_saved_successfully = @user.save
  if not user_saved_successfully
    raise PasswordExeception, 'activerecord.errors.models.user.attributes.password.too_short'
  end
end

def error_message_and_retry(error_message)
  flash[:error] = t(error_message)
  render :password
end

class PasswordExeception < Exception
end