Original Alignment: Chaotic Good
Suggested Alignment: Chaotic Neutral
This is perhaps the biggest case of mistaken alignment in the entire series. Coran shows very few redeeming qualities. Now of course, Coran is NOT an evil man. We don't see him stealing from the poor or killing innocent people, but Coran is too much of a selfish jerk to be Chaotic GOOD. When we first meet Coran he seems to be motivated entirely by greed and will threaten to leave the party if the quest for Wyvern heads is not given top priority. Any sensible Chaotic Good character if they discovered evil was afoot would change their priorities at least until the matter was properly settled before returning to their own pursuits, whereas Coran does not. Now other people have pointed out Coran treating women like objects and abandoning his children but while these are terrible things to do they alone aren't enough evidence in my opinion to warrant him not being good as every alignment is more complicated than it appears. However when you combine these two you get a man who is motivated completely by the self, which does not align with the typically selfless principles of Chaotic Good
Original Alignment: Neutral Good in BG1, changes to Chaotic Good in BG2
Suggested Alignment: Chaotic Good in BOTH games
Some people will make the argument that Minsc's alignment shifted due to the death of Dynaheir in BG2, but I suggest that Minsc's alignment shifted due the the head wounds he suffered prior to BG1. Others argue that Minsc should not be considered good at all due to his berserk actions against those that would refuse to aid him but I think such observations are ridiculous as this behavior is chalked up to Minsc's lack of judgement due to suffering obvious brain damage. Minsc is still good at heart but simply needs an guiding hand to keep him on the right track, he is still an excellent champion of butt kicking for goodness.
Original Alignment: Chaotic Evil
Suggested Alignment: Neutral Evil (I would also accept Lawful Evil, but this is pushing it)
Baeloth doesn't seem to have the bloodthirsty and carnal nature of Chaotic Evil. What he DOES have is the self-absorbed and practical nature of a Neutral Evil character. Baeloth is exceptionally intelligent, capable of weaseling his way into CHARNAME's group, doesn't constantly make suggestions but rather bides his time using CHARNAME in order to get stronger for his return to power and to get revenge on those who made him suffer. Baeloth is surprisingly polite and knows how to get along with most people and be tolerated by the rest even though it is clear he sees everyone as beneath him and thus actually fits into pretty much any party not led by a Paladin or some other LG or LN fundamentalist.