I don't know about legally -- I know Eagleburger was removed as Veep candidate by McGovern after the convention. But it is for all practical purposes too late...it would create party chaos -- the party would start coming apart, it would be a disintegrated mess and Hillary wouldn't be geared up to campaign. Even under the very best and most favorable circumstances, with Hillary all geared up having gone through primaries with a staff, Bush would still eat her up and spit her out. She'd have no chance, being such a divisive figure against a President with approval ratings over 50.
Kerry will do a Mondale -- take one for the team and keep the Party together to try again in '08. Heck, by doing that he'll get 40% of the vote or a little more and carry a handful of states. But mostly, he'll be playing the good soldier and keeping the party intact. That other way, they spin off into many factions.