You could use Find/Change to replace fonts.
If you want to use script you can use the property appliedFont which is read/write.
app.activeDocument.stories.everyItem().appliedFont = app.activeDocument.fonts.item("What Ever");
would set the the font of every story to the font "What Ever".
Text en Paragraph etc. also have the appliedFont property it is also Read/Write
Thanks guys, really helpful tips.