Other mistake: Chucky cuts Andy's Achilles tendon from under his bed. Throughout the rest of the movie, Andy should have a limp from such a severe injury. However, he has no trouble walking or running in subsequent scenes, nor does he show any signs of injury.
Other mistake: During the breakfast scene, Sgt. Botnick tells Andy he needs a trim on Wednesday, but he had just given him a haircut the previous day.
Other mistake: When Charles is thrown away and gets dumped in the back of the garbage truck, he screams for the garbage man who hears him and jumps in. There's no way he could have heard Charles screaming or the cadets hear the garbage man screaming from the marching field. Front loaders are loud and nearly impossible to hear screaming even near the truck.
Answer: The film makers wanted to make Andy older in the film so he could have a love interest and be in military school. At the time Alex was only 9, but they wanted Andy to be 16. So they recast the part.
Bishop73