You can use a mod to change in game fonts. Or you can replace the original game fonts with the fonts you want to use. Create the following folder:
Create copies of the font file of your choice, place them in the above folder, and rename them to the following names:
FRIZQT__.ttf (the main UI font - NPC names, player names, spell names, item names, buffs, quest log text, combat text, button text)
ARIALN.ttf (chat font, info text, small text)
skurri.ttf (default unit frame combat text)
MORPHEUS.ttf (mail text, quest log header text)
FRIENDS.ttf (friends list text)
Ansi Combat Font
Little tweak which enables a new default font to your UI. Only for styling purposes. Wastes almost no memory.
KryDamageFont is a easy and lightway addon to change your Damage Font.
tekticles is a simple set of fonts, derived from ClearFont2. I only ever used one set of fonts from CF2 (the Calibri set), so I ripped CF2 down to it's bear ass and stripped off all the extra crap. This addon is what's left..