The way I see it since comics are more of a medium of art, a way to convey a story, they are different than a portrait. Going back and changing Guernica is one thing, as it is supposed to only be one solitary piece of work. Comics, on the other hand, are a more dynamic medium, they are supposed to carry on and they can change over the years to best fit the times. It all equates to the story however. If the change drastically alters the story that is being told, it is bad, but if the change is only superficial and only with something like a character’s race or gender, whose importance to the integral persona of the character varies by character, then to me it does not matter.
Using your example of Speedy from Arrow. Arrow is an adaptation of the Green Arrow comics, and as such, it is allowed to alter things that would make the show better. Adaptations change things all the time and sometimes the changes are pragmatic and help the show and sometimes they are too drastic and they destroy the show. Since the show is still in its beginnings, it is hard to say what role “Speedy” will play, but from what I’ve seen with her, she could easily join her brother’s crusade and she already has a drug problem, which could lead her to becoming the Speedy we know and love. Just because Speedy is now a woman does not mean the core character cannot be there.
For those of you who didn’t want to read all that, in summation. Changing things like race or gender do not have to change the core tenets of the character and as such, in my opinion, are OK, as long as the material treats them with the dignity they deserve.