http://en.wikipedia.org/wiki/List_of_The_Fresh_Prince_of_Bel-Air_characters#Vivian_Smith-Banks
Vivian Smith-Banks
The character of Vivian Smith-Banks (Aunt Viv) was portrayed by Janet Hubert-Whitten from 1990 to 1993, and by Daphne Reid (credited as Daphne Maxwell-Reid) from 1993 to 1996. This change of the actress portraying her also causes Will to comment on the change of her appearance in the opening episodes of the new season. She is a professor of Black History and Literature at an undisclosed university, and in one early episode takes a temporary job as a teacher for a unit in Black History at Bel-Air Academy, the fictional upscale preparatory high school attended by Will and Carlton. She has three sisters: Helen, Janice, and Will's mother Vy (Viola).
Vivian's personality and role in the show changes over the run of the series. During the Janet Hubert-Whitten years she's a no-nonsense, forthright, career-minded woman who plays a part in the show equal to the character of Philip. After Maxwell-Reid took the part in 1993 and baby Nicky was added to the cast, the writers seemed to de-emphasize her former persona and generally portrayed her as a loving, supportive, non-confrontational homemaker with a smaller role in the show's plotlines.