You'd have to know what percentage of each party voted with their own party's candidate. The biggest shift would occur if each party's voters voted for their guy, and independants were the only swing voters.
Bush is supposedly leading independents by 3%. I've seen where Rats and Pubbies were about equal in voter registration nationally, so if the same number of Rats and Pubbies were crossing party lines (a big presumption), Bush would be up about 1%. Bush's people complained that the poll was leaning about 10 to 12 percent too democratic.
Bush has a much higher lead among Republicans than Kerry has among Democrats, on another thread someone said that if you balanced the parties equally, considering the percentage of support within the respective parties, Bush leads by six or seven percent.