IF - and I mean IF - he was able to sell it with legit test results on Biden's aneurism from 20 years ago, it could work.
He'd take heat from cable pundits - but no staunch obama voter is going to vote Mccain. And he'd only do it if he's losing independents - and damn those hilary voters would come flocking back. It'd be the breaking news story of the year.
You're suggesting (I think) that this would be a strategic decision by Obama rather than a genuine reason (if Biden had real health issues I'm pretty sure they'd keep it under wraps and have him step down after the election and their presumed victory).
There is no way the replacing the VP would increase Obama's chances. Any tentative supporters would run away fast.
BTW - did you read the snopes link? The only time a VP candidate was replaced prior the election was by McGovern in 1972 and he got stomped.
The cure is worse than the disease.