I agree with holding withering fire right now, because the election is 6 months from now, and because Kerry is shooting himself in the foot pretty well. But, I don't think the end of the DEM convention (which I assume is the event you consider as marking "technically nominated") necessarily marks the winner as the person who GWB will run against.
The DEM party holds to no rules, and will switch Kerry out in a heartbeat once it is prevailing belief (stoked by media propaganda) that Kerry cannot win.
See Toricelli/Lautenberg. Expand it to the presidential level. Either they get away with it, or there will be, ummmm, well, errrr, lots of angry Democrats. Use your imagination.
While a few would go along (the New Jersey supreme court certainly has set a precedent for itself with the replace of Torricelli with Lautenberg), it would be beyond the 'Rats reach to get enough states to go along to be able to win.
Moreover, the stink would kill them in any close state, especially if the name on the ballot was still "Kerry", but voters were supposed to vote for "Kerry" in order to get, say, Hitlary. Yeah, the corrupt and the radical leftists, but I repeat myself, would go along. But this would lose a majority of the "silent majority".