The Save commands default file format must be configured.
When users create new document files, Word 2013 saves them in the new Word 2013 .docx format. Ensure this setting is enabled to specify that all new files are created in Word 2013. If a new document is created in an earlier format, some users may not be able to open or use the file, or they may choose a format this is less secure than the Word 2013 format. Users can still select a specific format when they save files, but they cannot change default of this setting from the Word Options dialog box. This enforced user behavior ensures any change to the file format requires additional deliberate user interaction.