I want to update documents on my Mac that are in Microsoft Word format.
Other than getting Office for Mac, do I have other options that work seamlessly?
You'll have to judge what's seamless. You may want to look over the features for yourself, too. Here's a guide to help you in your search.
Products that I don't know well enough to discuss in detail:
I have owned Mellel more recently. I wanted its great support for right-to-left languages. I could see that the product is done well. But I never needed Mellel enough to give it a good workout.
Judging by their own sites, all of the above have Microsoft Word compatibility, but they generally aim for compatibility that is adequate rather than obsessive.
I recommend putting Microsoft Word users like me on LibreOffice although your sense of seamlessness may lead you to NeoOffice. They are among the notable descendants of the OpenOffice.org code that run on a Mac.
Their support for MS Word documents is far above "adequate," including a great variety of features that are used in MS Word documents. An example of what counts as a Word compatibility quirk in LibreOffice is a table that, although drawn in its correct position, is displayed within a non-printing box that overlaps the page margins. That may look strange, but it still works just fine. For years I have opened MS Office documents in LibreOffice, edited them, saved them in MS Office format, and sent the modified documents to MS Office users. Out of all of those times, there was only one complaint, one elaborate table did not display very well in Word.
Some of these products used to run under the X Window System rather than the Mac's Aqua graphic system, but now all of them run under Aqua.