The best way to change the letters "A" and "B" in places

There is a function that takes an argument in the form of a string consisting of "a", "A", "b" and "B".

Each "b" should be replaced by "a", each "a" by "B", and also with the lower case.

>>> swap('aabbbaa')
'BBAAAAABB'

Wrote such a weak code, but it works:

def swap(text):
    new_text = []
    for c in text:
        if c == 'a':
            new_text.append('B')
        elif c == 'b':
            new_text.append('A')
        elif c == 'A':
            new_text.append('b')
        elif c == 'B':
            new_text.append('a')
    return ''.join(new_text)