I have Word 2003 SP3 with all updates and the latest FileFormatConverters and its SP installed on several machines, all running W10-64 Pro. In all cases the machines got to W10 by running the W7>W10, and in all cases that ran smoothly. In some cases Word 2003 (well, Office 2003) was installed before the upgrade, sometimes after. It makes no difference.
In all cases, Word 2003 appears to run just fine EXCEPT that the docx converter (wordconvpxy) doesn't run. On one machine, I don't even get the message that says that wordconvpxy failed to start, it immediately just goes to showing junk. Of course, W10 Wordpad works fine with docx so I can get at the files that way, but that's not a very appealing solution.
The docx converter DOES work on W7-64 Pro.
Of course I have done reinstalls, did the MS suggested key delete/repair routine, etc. Nothing has any effect.
I can't find in Word 2003 any place where I can see that it knows that the converters are even there, and looking on my reference XP machine is no help.
It seems funny that a dumb little 25KB file will not work.