I just installed the Word Preview app from the store - it's fairly usable, but... when you open an existing docx file, it claims it is in an old file format, offers to save a copy in the newer format, or loads the file read-only. So it saves a copy as a docx file. Word 2007 can work with this file, but if you edit it and close it, opening with Word Preview gives the same file format warning.
Editing the file with Word Online does not have this effect.
Strange! Anyone else seen this? What sort of flag is it picking up in the Word 2007-edited file that makes it think it's an old format, even though it's docx? Whatever!