Is it possible to update an existing EC2 security group from CloudFormation?

Existing SecurityGroups can be updated using SecurityGroupIngress

For example:

"SecurityGroupRedisIngress": {
    "Type": "AWS::EC2::SecurityGroupIngress",
    "Properties" : {   
        "GroupId": "sg-123456789",
        "IpProtocol": "tcp",
        "FromPort": "6379",
        "ToPort": "6379",
        "CidrIp": "1.2.3.4/0"
    }
}