Billy character, was always more than just the first issue which painted that image of Billy. But Billy was a character who still stuck up for other people and tried to help them anyway he can:
The difference is his rougher attitude, which was dropped after the first 2-3 issues was shown to be an act. He was used to people letting him down and doing better on his own (especially based on his last four foster homes according to one line of dialogue of his social worker who doesn't care about him as long as he doesn't affect her pension). He's learned to detatch himself from the situation, until he met the Vasquez family. There is difference between a Billy who is 10-13 like most origins who's been on his own and still have a hopeful attitude, and a Billy who's been through the foster system until 15 and bounced from family to family to the point where he rather live in the subway. It's not simply just a matter of character development but Billy is a kid who cares more for other than he would want to admit. The foster home is also a plus, seeing them bounce off of Billy. I would say reading the graphic novel as a whole paints a better picture of Billy as a more nuanced character than just the one story. Especially with the new series coming out.
He was still someone who cared about people, even the villains despite what they did. Like in Justice League#22, where he trying to bury Black Adam in the place of his origin:
He's far from a nasty brat to be honest. His first action as Shazam/Cap, is to save someone from being robbed without a second thought. That alone speaks a lot about his character
Unlike what Kurt says, it doesn't ruin the characters or ignore the characters despite changing elements. Far from it. The origin may be the starting point but we don't know where they are going to next. It still gets to the heart of Captain Marvel in a lot of ways.