PLEASE HELP: Need to convert thousands of .doc files to .docx

I need to convert -- in large batches -- over 50,000 .doc files to .docx (true XML, not "Compatibility Mode"). It is critical to not lose formatting, tables, graphics, lists, etc.

I already have a script (droplet) that opens all selected .doc files and converts them, 1-by-1, to .docx. It works flawlessly, in that all formatting, tables, graphics, lists, etc. are maintained. However, the deal-breaker is that it takes forever (i.e., about 30-60 seconds per file) because it relies on Microsoft Word itself to open each file, convert it, save it, close it, and repeat with each subsequent file.

So, I am hoping that someone already has a script -- or can alter a similar one -- that quickly converts large batches of .doc files to .docx (true XML, not "Compatibility Mode") without losing formatting, tables, graphics, lists, etc.

PLEASE HELP ME!