Assuming the latest version of OpenOffice doesn’t work for these files…
My next course of action would be using the Win 10 machine with Word, or a VM with Win10 or 11 and the latest version of Word. Use MASGrave to trick M$ into considering it licensed if you need to.
Use a Powershell script to interact with Word through the COM object interface and automate opening Word, opening the file, saving it as a different filetype, and closing. Here’s a snippet of Powershell from Reddit for going in the opposite direction (odt to docx) for a single file. I wouldn’t try to do this through Linux, just suck it up and use Windows so you don’t have an extra layer of mess to deal with.
Going off M$ documentation of the save types enum, I would replace “wdFormatDocumentDefault” in that snippet with wdFormatOpenDocumentText or wdFormatStrictOpenXMLDocument, then test it with a single file to see which gives the output you need.
Getting all the files of the starting type from a folder can be done using Get-ChildItem. Store those in a variable and use a foreach loop over the initial file list.