Randomly replace certain words in python using a dictionary

Does anybody know how to modify this script so that it randomly changes the words when it finds them.

i.e not every instance of "Bear" becomes "Snake"

    # A program to read a file and replace words

    word_replacement = {'Bear':'Snake', 'John':'Karen', 'Bird':'Owl'}

    with open("main.txt") as main:
        words = main.read().split()

    replaced = []
    for y in words:
        replacement = word_replacement.get(y, y)
        replaced.append(replacement)
    text = ' '.join(replaced)


    print (text)

    new_main = open("main.txt", 'w')
    new_main.write(text)
    new_main.close()

Thank you in advance