iPhone Battery Replacement

[Cergman]

I have several iPhones, at least one of each generation in my closet. I have an iPhone 3G that's seen better days, although the front glass is not cracked. If I got a battery replacement service for $79, would Apple replace the entire phone or would they only swap the battery? Also, do they do any prior testing to verify the battery is not working correctly? The iPhone 3G maxes out at 4.2.1, so I'm not sure if that supports the new diags:// diagnostics.