Words, this is what the person is likely already doing, albeit manually rather than with a macro. The problem expressed is that doing this changes page and paragraph formatting. That is a result of formatting not using Styles and the use of manual line breaks or the Enter key to manage spacing. If the space bar is used instead of tabs or tables for horizontal spacing, columns will be all over the place. This use of the wrong tools for spacing text on a page will be highlighted by changing from a monospaced font to a proportionally spaced one.

Simply changing the font of text in a document directly (not through use of styles) is done most simply using Replace rather than using a macro. That can be done without changing font size, bolding, Italics, underlining or other formatting.