I really hope this happens! I'm not a comic reader, but I'm all for change. My view point on this stuff is change things but don't deviate from the source matieral too much, use the stories from the books as a base, but put new twists on them, update them to work better in todays movie going world. But only if they're going to work well in the context of the movie in the general audience eyes, even if it makes the die-hards a bit angry, the movies aren't solely for them they're for the 95% other people out there. See: Iron Man 3, which I'd say 50/50 here enjoyed the change and 95/5 in the general audience liked the way it all worked out in that movie.
From what I know changing this character from white to black or indian or oriental doesn't seem to alter the character at all, him and Sue can still be brother and sister in a few different ways which could be easily explained in a few sentences in the movie ("Oh sorry, that's my brother Johnny giving you a hard time. *odd look from who's being talked to*, yeah he was adopted when we were just kids, but we're just as close as any blood siblings.") And if this actor they want, brings to life all the personal characteristics of Johnny Storm then go for it. General Audience won't care for the change, these comics characters are just movie characters to them, they aren't familiar with the history and probably don't care to look it up.. they just want to see a good movie, and if the person portraying that role does an outstanding job then they should have the role.
I think the whole "he's been white for 50 years and that shouldn't be changed" thing is a bit selfish. Things from the comics are bound to change when adapted to film, and I think it's usually for the better.