Yes, a lot of people are going to yell and scream about this because they are racist.
But I do not think the OP is one of them.
The comic industry has done tremendous damage to itself over the years by trying to be trendy or shocking to increase sales. I read a ton of comics years ago but was driven away by rising costs, constant reboots, and multiple redundant titles. They kept trying to boost sales with gimmicks like the Death of Superman or Knightfall--yes it sold big numbers for the event but everyone bitched about how awful the follow-up was and they just forced the train back on track.
Now the hot topic is to drastically change characters by altering race, gender, or sexual preference (in addition to constantly returning from death--it's like a bad soap opera on that front). In a lot of cases it's met with a whopping "who cares?" since many of them have been fringe or lower-tier characters. However, when you start making wholesale changes to the big names--fan-favorites that kids grew up idolizing--it is going to rustle some jimmies. And it usually seems like it is being done for shock value, based on the writing (from what I have seen--I admit that I have not read many titles regularly for years and try to catch up by spot reading or summaries).
Is it necessary to make Captain America black? No, because the whole point of his character is that he is a white (blond) male who has historically stood for everyone's rights--regardless of race, gender, orientation, social status, etc. Civil War was a perfect example. I didn't read AvX but it sounds like an aforementioned example of crappy writing and getting away from a character's actual values and beliefs.
Should Thor be a woman? Hell, no--his freaking name is Thor OdinSON for crying out loud...
The forced diversity by changing existing characters is very awkward and lazy. The United States of America started out as a nation of white people (please do not start the Indian/Native American argument--that is a totally different aside). Over the years it has grown into a nation of great diversity--more so than any other country in the world.
Comics follow a similar path--both DC and Marvel were lily-white in their inception but instead of growing into diversity by adding new minority characters, they just rewrite the existing ones--no matter their historic importance.
Now I understand that this is an awkward analogy but please stick with me: what comics do is to write about someone like George Washington and make him black instead of writing about Martin Luther King, Jr. (also like the retcon of the "first" Captain America being black--very weak) Yes, the world has changed but don't address that by changing the past--move into the future and create new characters that have a chance to grow into greatness. Or tinker with it by doing so on a smaller scale. One example that I don't have a problem with is Ms. Marvel--I love Carol Danvers (and the old costume--thigh-high black boots are always a welcome sight) but she took the mantle of Captain Marvel and moved on in her life. Then they could create a new Ms. Marvel and introduce a Muslim teenage girl. Did they change the character? Sort of--but Carol is still Carol and doing her thing so this was done in a way that made sense for both characters and was done less controversially.
Sam Wilson is great. But to me he will always be the Falcon. Captain America will always be Steve Rogers--not John Walker, not Bucky Barnes, not Sam Wilson. If they want to ditch Falcon, have Sam develop into something else--don't force him into it by having Steve become weak (which we all know will get retconned/reversed in the near future anyway).
C'mon, Marvel and DC--you pay these big-name guys pretty good money. Demand some creativity from them and cull back the number of stories they need to churn out each month. We'd get a tighter product that would be less inclined to go for goofy gimmicks.
An interesting question (that would highlight additional racism and excuses for defending it from some people) would be--what if iconic black characters were suddenly made white? Black Panther? Storm? Let's go into other media--Shaft? Many would say that the fundamental basis for those characters is that they are black--but is that any less racist? Something to consider when attacking people as being bigots because they don't want to see characters that they identify with or idolized changed on such a major scale...don't jump to conclusions.
For the record, I liked the Sam Jackson-ification of Nick Fury--Ultimates was cool and I thought the whole process was pretty funny.
And to whoever asked why no one likes Richard Rider? I remember him from my youth and he sucked back then--don't know what they've done with him lately but a lot of the old folk will probably always see him as a flaming bag of crap...sorry--not necessarily fair, but kind of how it is sometimes.