import re
pattern = r'("[^"]*")|:'
s = 'x:123; y:"z:456"'
res = re.sub(pattern, lambda x: x.group(1) if x.group(1) else "=", s)
print(res)
aW1wb3J0IHJlCgpwYXR0ZXJuID0gcicoIlteIl0qIil8OicKcyA9ICd4OjEyMzsgeToiejo0NTYiJwoKcmVzID0gcmUuc3ViKHBhdHRlcm4sIGxhbWJkYSB4OiB4Lmdyb3VwKDEpIGlmIHguZ3JvdXAoMSkgZWxzZSAiPSIsIHMpCnByaW50KHJlcyk=