I checked out somewhere around the time they revealed Carl was actually Icelandic, which only explains why his full name is Carl Carlsson (remember, in Iceland people do not have family surnames, and instead adopt one of their parent's names and add the suffix 'son' or 'dóttir' as a surname). Guess what, it was also a perfect excuse to shove in a Sigur Rós cameo, back when they were kinda-sorta popular in the mainstream.
Similarly, I don't see why would they need to change Dr. Hibbert's voice other than "to be voiced by a black guy for more accuracy." His whole schtick is to be an average MD of varying degrees of competence and with a penchant for trolling his patients.
Oh yes, Apu's "disappearance" due to some pajeet complaining about the character being supposedly racist was the subject of much discussion, and perhaps the most exposure in the public eye the Simpsons had in ages. I seriously don't see how Apu could be racist - Stereotypical, maybe, but quickly undone by Apu being well-fleshed out. Sadly, the pajeet won, Apu was relegated to the background, and even Hank Azaria agreed to never voice him again due to those astroturfed concerns.