Ryu retains his familiar costume, a white gi, red sparring gloves, a red headband and a black belt (that is beginning to fade). Ken is still primarily wearing red but his top is pulled around his waist. He has a black sport top on now with red trim. His gloves have gone from yellow to black and he is now wearing black sleeves on his feet. Combined with his extra long hair this is a big shift from game to game and even for the entire series.
A few things that concerned the fans were valid. Ken no longer had a primary color associated with him, his two colors were complimentary but they reduced the visual impact he once had. The other thing that concerned fans were how the red and black combination had been seen previously on other blonde American fighters. Primarily Jacky Bryant from Virtua Fighter and Paul Phoenix from the Tekken series. Both were considered derivative of Ken but now the opposite appeared true.
Something else that may have concerned audiences was how similar the makeover was to fan concepts for Mel Masters, the son of Ken, if he were in the series. Of course he was expected to wear red to honor his father but would have added his own touch as well. Perhaps a black top or black tape on his hands and feet. Black was one of the few colors that worked to compliment or contrast red which was why it turned up on so many characters. These fan ideas were many years old and I'm sure many wondered if the artists at Capcom had been keeping tabs on what interested audiences, especially gamers in the West.
There had not only been heroes that were dressed in red and black. One of the great all-time villains from SNK continuity wore a costume very similar to Ken's updated look. Rugal Bernstein was the organizer of the original King of Fighters Tournament. The game debuted in 1994 and was a revolutionary title. It's use of tag teams was a refreshing change of pace from the typical 1-on-1 battles. Rugal himself was very powerful, as was the case with all SNK boss characters. He was also tall and very strong, giving him an advantage whether opponents were close or far away. The developers made good use of his reach by assigning him many kick-based attacks.
Ken and Ryu had always had the "hurricane kick" in their arsenal. Ken had specialized punches and kicks that actually set his opponents on fire going all the way back to Street Fighter Alpha / Zero in 1995. With his updated look in Street Fighter V he had some changes made to his library of kicks. Several of them now had sweeping arches. These new kicks reminded me greatly of the kicks Rugal would perform. Granted they could have been coincidence. I get the feeling that in the subconscious of the new designers they remembered Rugal and Ken from the same era and combined them into this new fighter. But that's all speculation.
I can understand the concern that long-time players have with the character. He's been presented in one way for so long that the new Ken may be completely "unplayable" in the eyes of traditionalists. Capcom would not do that to one of the main characters. A supporting character like Birdie on the other hand may have an even more dramatic look and control shift. The thing is that Capcom pioneered the idea that fighters could change from game to game. Street Fighter II and Alpha were worlds apart. Ken was very different between the two. Other studios saw a cosmetic change as a way to keep things fresh. SNK experimented with this in the King of Fighters series. A man costume and an alternate costume could both be considered canon. Ryo's alternate costume designs, including changes with his hair and facial hair, from a few years back were not unlike Ken's makeover.
In other cases SNK was able to completely redo a character's look and still have it work within the game series. Perhaps the makeover was because a lot of time had passed. Such was the case with Terry Bogard between the original Fatal Fury timeline and the events of Garou: Mark of the Wolves. Terry still played like Terry despite his new look. It was accepted by King of Fighters Maximum Impact that the main character could have two distinct looks. With the news that some of the original costumes will be available as DLC in Street Fighter V then I have a feeling that was what Capcom was going for with Ken. Some time has passed between Street Fighter III and IV so all the characters are expected to look a little different. So again, I understand the concerns of audiences young and old with the new look of Ken but the game is a long way from being finalized. Let's not forget about the eventual Ultra and Super versions that will follow and continue to shape the costumes and evolution of each character. If you have any concerns with the direction of the series I'd like to hear them in the comments.