Expecting bidirectional interoperability between very old, EOL formats, and very new cutting-edge technology does not make sense.

Making a decision to avoid migrating to a format that is obviously replacing an older format, for nearly a decade, should have been a decision with easily foreseeable future consequences.

The future consequence is you may finally have to install the '.docx' import plugin for those ancient copies of Office 2003. Take it as an opportunity to make a PowerPoint about how much money was saved by installing the free import plugin, vs. cost of new licenses.

Alternative: continue doing business as you have been, ignoring new technologies as you have been doing. Full interoperability between cloud tech and old, replaced file formats is not a safe thing to assume will happen. If lack of full, native, bidirectional interoperability between legacy 'doc' / 'xls' formats is having enough of an impact on anyone's business that it's "a very big deal", someone is doing something wrong.