Lestat changed Louis to get his house and money. At least that's what Interview said, I think The Vampire Lestat tried to come in and retcon it some to make him not such the douche bag...but honestly I liked bad guy Lestat better then super Mary Sue Lestat. But then Louis, not Lestat, is my favorite of all the characters...and he gets treated like such a bitch in later books. He shows up in The Vampire Lestat, The Queen of the Damned, maybe Tale of the Body thief, and he is at least Mentioned in Menoch the Devil and The Vampire Armand....but in almost all of these he is just another in a line of people who are constantly kissing Lestat's ass. He has never been as compelling as he was in Interview and most of the time he only gets a passing mention in those later books.