I'm not the one who changed it. It's changed over time, the way the
rest of the language changes.

true. Just like when I was growing up, "that's a bad shirt you have on"
or "that's so bad" meant the opposite of good.

Now the exact same sentences means the same as really good.

depending on who's saying it and whether they mean 'really good" or mean
"the opposite of "really good" when they say "bad'.