if I upgrade to Big Sur will it support my MS Word 2008 for Mac?

No, it will not. Microsoft Office 2008 is a set of 32 bit programs and will not work on macOS Catalina or later which only run 64 bit programs.

You will need to buy an newer version such as Office 2019 ro 365 or an alternative application set like Libre Office or Apple{s own Pages, Numbers and Keynote. Both of which are free.

The easiest way to tell is try and install it and see. I have my doubts as Office 2008 is ancient by software standards and Microsoft stopped supporting it years ago.