If you are into fiddling about with font families, sizes and/or colors on each and every post you make by using the Text editor (HTML editor) then you can change it in the body text in posts and pages - no upgrade is required. But for changing it in Titles and widgets then you need to purchase an upgrade.
These are 3 ways to change fonts on WordPress.com blogs.
(1) Changing font families, colors and size in text in posts or pages either post by post or page by page by coding into the Text ie. HTML editor. http://wpbtips.wordpress.com/2009/07/11/formatting-text-pt-5/
(2) Changing font colors in text in posts or pages either post by post or page by page using the #4 icon (Select text color – change the text color) in Row 2 the Visual editor. http://en.support.wordpress.com/visual-editor/#row-2
(3) Changing all font families, colors and sizes throughout the whole blog by purchasing an annually renewable Custom Design upgrade and using Typekit Fonts with Staff support.