Before Lee embarked on his Gettysburg campaign Longstreet had argued several times that he should be sent out west to reinforce Bragg but Lee was reluctant to let one his most able subordinates go and so Longstreet remained in the East.
After Chancellorsville, as Grant was progressing towards Vicksburg out West, Longstreet once again tried to persuade Lee to allow him to go west to reinforce Bragg in Tennessee against Rosencrans, believing that if he could drive Rosencrans toward the Ohio River he could force Grant away from Vicksburg.
Again Lee refused, even more weary to lose his best subordinate following the death of Stonewall Jackson.
So what if, for whatever reason, Lee allows Longstreet to head out west in 1863?