exactly.
these characters, especially comic book characters ( since comics are a VISUAL medium ), have 2 identities. A visual identity ( how they look ) and character identity ( how they behave, their character type, etc. ).
If you change one identity, at least try to retain as much of the other one so that the character still feels familiar on some level. If you change both identities, then you've pretty much made a whole new character.
In Dean Cain's case, aside from him being half-Asian ( which, tbh, I didn't even know until I read it online much later - I didn't know he was half-Asian when I 1st saw him in Lois and Clark ), his Clark/Superman pretty much retained both the visual and character identity of the character.
let's take another example, MJ from the Ultimate cartoon show.
Ultimate show MJ retains the visual identity of the character - white, red hair, green eyes. So, at least she looks like classic MJ.
Her character identity was changed, though. They made her an aspiring journalist who wants to work for the Bugle. They basically transposed Peter's connection to the Bugle onto MJ. They basically transposed Lois Lane onto MJ.
So, from a character type perspective, she seems like a different character than comic MJ. But, hey, at least, she looks like MJ. You can immediately identify her as MJ.
Now, just imagine if they changed her visual identity, too. What if they made her blonde like Gwen? Or a short-haired brunette like Betty? What if they made her dress all punk-like with a Mohawk and nose ring? What if they made her Asian?
If they did any of that, they'd not only change her character identity, but her visual identity as well. And then, she might as well just be a different character, because she would neither look nor feel like MJ.