Batch conversion of xlsx to xls via CLI?
I've not tried this with .xls(x) files per say, but I have used LibreOffice to batch convert .doc(x) files to .pdf using the convert-to option.  OpenOffice has the same options, afaik.

Check the help option for your app.  In my case it was:

/Applications/LibreOffice.app/Contents/MacOS/soffice -help


In particular, for my switch to .pdf files, I did something like this:

/Applications/LibreOffice.app/Contents/MacOS/soffice -headless -convert-to pdf:writer_pdf_Export *.doc


The -headless flag just prevents the whole GUI from loading.  There is likely a similar output file extension and filter (the pdf:writer_pdf_Export) for your .xls(x) case.