Microsoft
My solution does not require hacking. It's just a macro that I created by pressing the Record Macro button and then slightly altered to avoid a dialog message. It just automates the Compare Documents function and adds a button to the Ribbon. This approach is regularly used to automate functions, and the instructions I provided require no advanced knowledge of Word. Give it a try and if you get lost, I'll be happy to help.
In terms of downgrading, depending on how much older the version of word you are installing, you might lose certain functionality, and it's not something I would recommend, but I doubt you'd lose any files that you've created. You could install an non-365 version of Word and cancel your 365 version while still continuing to work with your files as normal.
