American Express updated their online banking interface and provides an XML-based QFX format that the GnuCash updater cannot import. This Ruby script converts the new format back to the QFXSGML format that GnuCash knows how to import. Experimental; use at your own risk. It works for me.

amex-ofx-downgrader.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

# Convert new American Express QFX files (since June 2020) to older SGML format
# that GnuCash can import.

require 'nokogiri'

def show_usage
  warn 'Usage: amex-ofx-downgrader.rb /path/to/source/file.qfx /path/to/output.qfx'
  exit 1
end

show_usage unless ARGV.count == 2

source_file = ARGV.first
unless File.exist?(source_file)
  warn "Input file #{source_file} does not exist."
  show_usage
end

output_file = ARGV.last
if File.exist?(output_file)
  warn "Output file #{output_file} ALREADY EXISTS! Wont overwrite."
  show_usage
end

doc = Nokogiri::XML.parse(File.read(source_file))

File.open(output_file, 'wb') do |out|
  out.puts <<~HEADER
    OFXHEADER:100
    DATA:OFXSGML
    VERSION:102
    SECURITY:NONE
    ENCODING:UTF8
    CHARSET:1252
    COMPRESSION:NONE
    OLDFILEUID:NONE
    NEWFILEUID:NONE
  HEADER

  out.puts '<OFX>'
  out.puts doc.xpath('//SIGNONMSGSRSV1').to_xml
  out.puts doc.xpath('//CREDITCARDMSGSRSV1').to_xml
  out.puts '</OFX>'
end