I really hope Chang Heng is noticeably different in terms of personality when he returns. He was already somewhat different but focused on acting 'his age', but I would think that just his temperament and natural responses to things would be wildly different after hundreds of years.

Cheng Heng is going to be absolutely terrifying in the real world as the story continues. Beyond just the effect that cumulative training has already begun to show, he'll have entire lifetimes of experience in combat and leadership roles. I hope either a group notices this and sweeps him up to make use of it or he puts it to use building his own force from the ground up with what I can only assume will end up being almost centuries of life experience after the next few simulations.