Enable Developer Mode in Word and Disable updating format.

If you have an old work document and you need to be able to save and edit the file without it updating, thus changing the format, this is the fix:

On the File tab, go to Options > Customize Ribbon.

Under Customize the Ribbon and under Main Tabs, select the Developer check box.

Disable auto update:

Click on Developer tab, document templates, and make sure auto update styles is not checked: