I can't think of a plausible cause, so let's just say a wizard did it. Would it cause a faster fall of the Confederacy or would it somehow give them a better chance at independence?
Longstreet had wanted to go relieve Vicksburg after Chancellorsville, but Lee didn't approve, would Jackson approve his request though? (On that note, if Longstreet ended up taking command, how much of the army could we expect him to send to Vicksburg's aid?