Let's say Jackson decided on a whim that he didn't really want to go out to check out the state of his troops that faithful night at Chancellorsville.
What would change?
Well, for one, Gettysburg would have certainly been a Confederate victory. It was lost to them for the main reason that Ewell failed to take a key hill. Undoubtedly, Jackson could have taken it. But how much of a victory would it have been? Would it be something like First Manassas, or more similar to, say, Second Manassas? Personally, I think it would've been slightly more slim victory than the usual Lee special, seeing as how the AotP was larger than ever before, and beginning to get the hand of their cavalry for once.
Still, the implications of a medium sized Confederate victory at Gettysburg would be enormous. Lee would continue to be able to march through Pennsylvania and deal enormous material damage.
However, Lee isn't Sherman. This would hardly be a March to the Sea, for the main reasons that Lee probably wouldn't destroy all towns in his path, and that he would have a large army opposing him. Regardless, many towns would be drained to supply his army as he doubtless shifted continuously eastward. Grant would finally be put in place as the new head of the Army of the Potomac, while Lee would continud rolling their right flank slightly in his push to Philadelphia. Finally, the Army of the Potomac was directly between the Army of Northern Virginia and Washington.
Grant would advance forward in an attempt to surge past the Confederates as their supply trains strained to supply them, and stop them before they reached Philadelphia.
Lee, unsurprisingly, anticipated a move similar to this, and shifted his forces to a series of defensive hills surrounding Upper Providence Township. Jackson, having immense artillery expirience gained from leading the VMI artillery squad before the war, ordered the cannons to be position in the prime locations near the center of the hills.
Knowing Grant, it seems likely that he'd swarm his forces forward in the way that he did in virtually every one of his battles. It could easily be assumed that a situation similar to Cold Harbor could occur, only with much greater repercussions. With a larger army available to both side, the casualty rate would be far larger, which would have a more profound effect on the AotP than the QoNV. Grant would more likely than not become yet another general in a string of Union generals humiliated by Lee.
Now deep in Pennsylvania, and near Philadelphia, and no army nearby to oppose him immediately, Lee, even with rickety supply lines, would probably reach Philadelphia and sack it. Utterly sack it. An orgy of looting and burning would rip through the city, causing massive material damage and seizing enough supplies from the Union warehouses to keep the army fed and clothed for months. Collassal supplies of artillery, horses, and weapons would be incorporated into the impoverished, barefoot, ascendant army. Critical railroad junctions would suddenly be cut off. Confederate engineers would melt and bend huge stretches of railroad in the area, rendering them irreparable.
Washington would be in panic. The second largest city in the nation would be gone. The Army of the Potomac would be in ruins. Washington was under direct threat. They called for even more men to join, desperate to create an army big enough to crush the AoNV.
Meanwhile, as the AotP slowly began to regather itself and reorganize, Lee suddenly smashed into their right in Downingtown. He began to rapidly roll them back, with Jackson spearheading the attack. It was a rout.
The Union army, then under the command of Meade, would have to pull their left back and withdrawal all the way to the safety of Maryland. Now content with the safety of his army to push north unmolested, Lee quickly set out to advance northward on a quest to ravage as much of the north as possible, living off the land as they went in the hopes of forcing a Union surrender.
By my estimates, it'd be similar to Sherman's March to the Sea, only not quite as destructive. As the AotP built back up in Washington, Lee would push north and begin draining supplies from Pennsylvania.
Would he reach New York State? New England, even? Would this be enough to actually win the US Civil war?