If you use any packages that change the font, those packages will change the default of the same type. For instance, using the Bookman font (which is done by using this command in the preamble: \usepackage{bookman})makes the default Roman font Bookman, but leaves the sans-serif and monospace fonts alone. Similarly, using the Helvetica font (done with this command in the preamble: \usepackage{helvet}) changes the default sas-serif font to Helvetica, but leaves the Roman and Monospace fonts alone. When changing fonts, you can do so like this, using the command and calling the particular font, or you can change all of the default fonts at once with the following commands.