Here what I do. Maybe this approach has not occurred to you before, but I'll put it out for anyone who cares about open standards and open formats.
When I receive a document in OOXML format I load it, make my changes and save it back into a free format like ODF. Remember, Microsoft Office since Office 2007 supports ODF. The more recent versions support it quite well. I *do not* encourage the use of OOXML or other closed formats.
If they are using an older version of Microsoft Office, say Office 2003, then they are not producing OOXML at all. So that is not an issue. But if you want to be absolutely safe, just save as a DOC/XLS/PPT binary format, which is widely compatible with all versions of Microsoft Office, OpenOffice, LibreOffice, KOffice, WordPerfect, etc.
It is a little bit disturbing to hear LO fanboys touting the virtues of writing OOXML files, as if this is a good thing. It isn't. Interoperability should be the goal, not promoting Microsoft's file format.