when after a person becomes offically a Marine and goes to his MOS,can we exchange our MOS if can how long would it take.and on the Crucible what do you do besides staying up for about 54 hrs.thank you for your help

Normally changing an mos can occur if you re-enlist to change that mos.
doing a lateral move while n your first four years is very rare but can be done if you have a specific skill that is needed.
Other mos changes can occur though. If you satrt out as 0311 infantry and become a recon bubba then you get the new mos designator for recon.
Some fields will let you go through advanced training that will give you a secondary mos designator