I'm fine with Marvel switching up race and ethnicity of secondary characters. A lot of these comic book characters were created before the civil rights movement so there either were no blacks or they were extremely stereotypical. Seriously, check out early incarnations of Luke Cage/Power Man.
With that said, I'd agree that they should try a bit harder to include a more diverse palate as opposed to basically just making everyone black. Off the top of my head, the only switch I can think of that wasn't white-to-black was the Asian warrior dude from Thor.
.. Speaking of all of this Avengers and Galactus/Thanos stuff, these next few movies would be perfect timing to introduce guys like Luke Cage and Black Panther. Without being able to get the rights to Fantastic Four and the Silver Surfer, I'm not sure where Marvel is planning on getting the firepower to take down a guy like Thanos, especially powered by the Infinity Gauntlet. They did a pretty good job of showing off Stark and Banner's scientific acumen but a guy like Black Panther could fill in for Reed Richard's brilliance in a pinch.
"I don't know how to put this...but I'm kind of a big deal"