You can't. It's all network side. You can only drop it back manually to 3G in the network settings but you'd find the speed of that intolerable.
Edit - Actually, I found this
searching through my thousands of S2 bookmarks. I'm not sure if this will do what you're asking as I've not been through it thoroughly/tried myself as I don't need to, but I think it's probably your best shot at achieving what you're after.