I don't know if there is a script to do this, so here's one I just made up. It's assuming you are using CS3 (you don't mention a version), and there are tiny differences between ID versions, so it might not run straight away.
for (var i=2; i<app.activeDocument.paragraphStyles.length; i++)
app.activeDocument.paragraphStyles[i].appliedFont = "Helvetica";
app.activeDocument.paragraphStyles[i].pointSize *= 0.80;
if (app.activeDocument.paragraphStyles[i].leading != Leading.AUTO)
app.activeDocument.paragraphStyles[i].leading *= 0.80;
It skips the first 2 paragraph styles because those are '[None]' and [Basic Paragraph]', and, reportedly, it's bad karma to mess with those.
The first line inside the loop simply sets the font. An additional style specifier (for example, "Regular", "Bold") is left out because you might have something already there. Notice that if you try to change "Helvetica Bold" to, say, a Frutiger, it will not be correct first time round -- Frutiger does not have a "Bold", it's called "65 Bold" for that family. Well, you can't win'em all.
The second line takes the current defined point size and multiplies this with 0.80, resulting in a new size 80% of the previous one.
That's it! Cheers, you've saved me a hell of a lot of time and effort. Many thanks dude!