Q, you asked a question upthread about African beliefs about cats.
While this doesn't address your question about cats changing to human form {or specifically to the form of witches}, here's a quote from http://www.goddessgift.com/goddess-myths/egyptian_goddess_bast.htm

"Egyptian goddess Bast, Goddess of Protection and Pleasure:

"Bast, Egyptian goddess of sensual pleasure, protector of the household, bringer of health, and the guardian saint of firefighters -- she was the original mistress of multi-tasking!

Also called Bastet or Basthet, the goddess Bast is widely known today as the "Cat Goddess". Legend has it that, by day, Bast would ride through the sky with her father, the sun god Ra, his boat pulling the sun through the sky.

Ever watchful, she protected Ra from his enemies. Thus she became known as the Lady of the East, the Goddess of the Rising Sun, and The Sacred and All-Seeing Eye.

But by night, she was a different creature entirely! Bast transformed herself into a cat (renown for its superb night vision) to guard her father from Apep (also known as Apophis), a serpent who was her father's greatest enemy.

Ra's priests burned wax models of the snake and wrote his name with green ink, trying to put a "hex" on him -- but to no avail. Finally, with her cat eyes shining in the dark, she managed to kill the evil serpent.

Credited with killing the vile Apep, the goddess Bast ensured the warmth of the sun would continue to bless the delta of the Nile with fertile soil and abundant crops and was honored as a goddess of fertility.

Because of her all-seeing sacred eye (called the utchat) that magically saw through the dark, Bast is one of the few sun goddesses that is also classified as a moon goddess...with her glowing cat's eye reminding us of the moon that it reflects".

-snip-

Information about Egyptian religion and other cultural components is relevant to a discussion about beliefs and customs in certain other African ethnic groups because-as I'm sure Mudcat members are aware- Egypt is in Africa and because a number of East African, Central African peoples are of Nilotic and not Bantu ethnicity.

For instance: "The main groups of tribes [in Kenya, East Africa] are the Bantu who migrated from western Africa, the Nilotic people who originated from Sudan and the Hamitic group, who were mainly pastoral tribes from Ethiopia and Somalia. The main tribes are Kikuyu (21%), Meru (5%), Kalenjin, Luyha, Luo (14%), Kisii, Kamba, Swahili, Masai, Turkana".
http://www.africaguide.com/country/kenya/culture.htm

-snip-

I've also read that specific West African ethnic groups [Akans ? Yoruba? ]have traditional beliefs that they migrated from Egypt an/or the Sudan. However, I'll have to look for online documentation or other documentation of that at another time.

I recall reading about African & Afro-Caribbean beliefs in cats changing into malevolent beings [not so say that real witches as in wica are malevolent].

However, I'll have to look for those online and off-line resources and post something on that another time.