This is probably a lot more than you feel like tackling, but once completed, you would find the changes you want to implement much easier.

The problem you are running into is that you have a number of different styles that are being applied. You need to redefine your base styles in the template being used for your documents (probably normal.dotm).

First, I would be typing most of my text using the Body Text style instead of the normal style. The Body Text style might be based on the normal style.

What you want is to structure the styles you will be using to all be based on a single style, say a custom style called basestyle. If you do that, and you change the font in the definition of basestyle, you will change the font in all styles that are based on that style so long as the style definition of the subsidiary style doesn't change the font.

Note, this is done by modifying the style definition, not just by selecting the text and changing its font. If you have your computer set up to change style definitions whenever you format text, you probably want to change that setting. See Formatting Applied to One Paragraph Affects Entire Document.

For more on styles see Understanding Styles in Microsoft Word - A Tutorial in the Intermediate Users Guide to Microsoft Word.