replace iphone 1st gen battery w/ iphone 3g battery?

i searched for this topic but i did not find an answer so sorry if its been asked before! anyways...since the new iphone appears to have much better battery does anyone know if those batteries will be able to replace old batteries in the "old" iphones? i was wondering if the new batteries were a different shape and size due to the redesign of the iphone. just wanted to know if the new batteries were an improvement in the hardware and if it could go in the old iphones. thanks!