There are two file extensions, but with every new word version there was new features, and thus new stuff that had to be represented in the saved file. If you only went forwards and didn't skip to many versions, and remembered to save the file again in the newer version you'd be fine. but as the mastodon thread says, anything from before a certain date won't open at all, and more complex documents from the 90s and early 00s might have surprising issues in modern word.