I might be wrong about this but I would think that a 301 redirect from the page missing in the SERP to the new page should pass on the PR to the new page.
Because the 301 should be indexed fairly quickly by Google the new page may just jump way up for the old pages keywords.

Eventhough onpage SEO is somewhat important, I have had pages rank in the top places when the actual keyword wasn't even onpage. It was the off page links that kept the page up in the SERPs.