When we first meet Kambei, he's getting his head shaved, in preparation for a daring rescue that, had it happened in Hollywood, would probably involve swinging through a window and uttering a vaguely clever one-liner before shooting the bad guy right between the eyes. He's disguising himself as a priest to get close to a bandit who has taken a child hostage: the old "hey, I'm just a harmless monk WHO CAN ICE YOU WITH MY HIDDEN KATANA!!!" routine.
But there's more to the act than just a disguise, and there's more to the shaved head than just looking like Michael Jordan. Buddhist monks cut their hair off to reflect their devotion to a simple and humble life. But samurai didn't. They wore their hair in top knots, or "chonmage" in Japanese terms, which actually helped them keep their helmets steady. It was a status symbol: a sign that they belonged to the nobility and should be afforded considerable respect.
Don't believe us? Just look at the faces of the people watching the shaving. They look like they're going to be sick to their stomachs. It's like he's cutting off his own hand!
By shaving, he's demonstrating his willingness to let go of a fundamental part of his identity, and suffering a huge loss of status in the bargain. People aren't necessarily going to get out of his way when he walks by now. They might give him more lip or even pick a fight with him if they take a mind to (though considering his swordsmanship, that's probably not a good idea). By shaving his head, he's getting ready to eat a whole lot of crow, which is why he's looking kind of grim about it.
More importantly, he's signaling that his status as a samurai is more fluid than it should be. He's giving up a part of who he is, and in essence saying that the code he's lived by is no longer going to dictate his every move. He's visibly losing a part of himself, and even if the hair grows back, the act itself can't ever be undone.
So why's he doing it? Well, there's a kid who needs saving and if losing his hair is the way to do that, then that hair has got to go. He's sacrificing his status and honor because the circumstances dictate it. Hey, it's what's gotta happen to save the kid, right? He's willing to bend his code to accept reality.
As we've said elsewhere, this is kind of a big deal coming from a culture that values honor and loyalty above all else. Their heroes embraced principles instead of realities, and would happily die rather than compromise their beliefs. But that all changed when America dropped an atomic bomb on them. They needed to accept that they had been defeated and find a way to move forward. Kambei's shaven head is being shown as a big deal, something that no right-thinking samurai would ever consider. But it's accomplishing something in the real world that just might be a little more important.
It's a tough pill to swallow, but if that guy on screen can do it, then maybe Japan could do it too. They could, they did, and some might say they came out of it a much better country as a result.