Longstreet’s proposal was rather vague in many respects. It was supposedly a sweep of many miles, necessitating a change of base for the Army of Northern Virginia. The plan made good sense in abstract, but was utterly impractical to execute.
Firstly, Lee does not even have a direct road south from Gettysburg, the southernmost road (Emmitsburg and Taneytown) were in Union hands. The best road available in Lee’s situation is the Fairfield road, which leads the AoNV southwest and is in plain sight of Buford’s cavalry division. In addition, Meade has the southernmost road, increasing the chances of Meade beating Lee to the south.
Secondly, the cavalry brigades with Stuart have yet to return. Although Stuart left a few cavalry brigades with Lee, these cavalry brigades were used to cover the flank and rear of Lee’s army and Jenkins’ brigade (the cavalry brigade brought to Gettysburg) was inexperienced in conducting reconnaissance. Lee needs Stuart back to screen and scout for the offensive movement.
Thirdly, it’s the dreaded L word, logistics. Lee would need to adress his supply lines and reroute his supplies. Planning for such a matter would take an entire day, let alone actually executing the task. OTL it took 24 hours for the first wagon to get out of Gettysburg after Pickett’s charge on July 3rd.
Supposing that Lee ignored all of these issues and attempted it, Meade would realize that something is up. Meade is already advantaged by the fact that he has already posted cavalry brigades to watch his left flank for any Confederate movement. By the time any Confederate offensive kicks off, Meade has already concentrated his army and is prepared for any moves by Lee. The result of Longstreet’s proposal depends on what happens next, but I think it’s safe to say that it won’t be much better to Lee’s decision to attack on July 2nd.