A script to convert rem values in files from one base font size to another. Works for me but ymmv.

rem-conversion.rb

#!/usr/bin/env ruby

require 'fileutils'

REM_PATTERN = /([\d.-]+)rem\b/

current_size, target_size, *file_paths = ARGV

def trim_num(num)
  num.to_f == num.to_i ?
    num.to_i :
    num.round(2)
end

def convert_rems(size, from_base, to_base)
  (size * from_base) / to_base
end

def with_files(paths)
  paths.each do |path|
    path = File.absolute_path(path)
    temp_path = "#{path}.temp"

    File.open(path, 'r') do |file|
      File.open(temp_path, 'w') do |temp_file|
        yield file, temp_file
      end
    end

    FileUtils.move(temp_path, path, force: true)
  end
end

with_files(file_paths) do |old_file, new_file|
  old_file.each_line do |line|
    new_file << line.gsub(REM_PATTERN) { |match|
      rem_size = match.to_f

      new_size = convert_rems(rem_size, current_size.to_f, target_size.to_f)
      new_size = trim_num(new_size)

      "#{new_size}rem"
    }
  end
end