Making one of the big guns gay would be fine, depending on how they handled it, but it wouldn't be as interesting to me because there are already gay superheroes in the present you can tell those stories with, so there's no reason to retcon one of the modern superheroes into a gay man.
Retconning one of the founders of the JSA into a closeted homosexual allows them to explore that idea in the 1940s, which is a story that no current character would be able to explore. Yeah, they could have retconned a newly created Golden Age superhero into the JSA, but I don't think that would have the same impact because that character didn't actually exist at that time. He'd very likely end up being a footnote like Triumph, the gay superhero that was retroactively inserted into the founding JLA.
The thing is, this doesn't change any of the previous Alan Scott stories. In the previous incarnations of the DCU, you can still have a straight Alan Scott. That version of him still exists out there in the Metaverse. But, in the current DCU, this Alan Scott is gay.