Terraform AWS - How to change default route to an existing route table from nat-gateway to ec2?

Question

I have a private subnet with default route targeting to a nat-gateway. Both were created by terraform.

Now I have another code to raise an ec2 to use as NAT in my VPC (as cloud-nat-gateway become very expensive). I'm trying to change the default route in my rtb to this new ec2 and getting the error below:

Error: Error applying plan:

1 error occurred: * module.ec2-nat.aws_route.defaultroute_to_ec2-nat: 1 error occurred: * aws_route.defaultroute_to_ec2-nat: Error creating route: RouteAlreadyExists: The route identified by 0.0.0.0/0 already exists. status code: 400, request id: 408deb59-d223-4c9f-9a28-209e2e0478e9

I know this route already exists, but how to change this already existing route to a new target? In this case my new ec2 network interface?

Thanks for your help.

Follow are the code I'm using:

#####################
# FIRST TERRAFORM
# create the internet gateway

resource "aws_internet_gateway" "this" {
  count = "${var.create_vpc && length(var.public_subnets) > 0 ? 1 : 0}"

  vpc_id = "${aws_vpc.this.id}"

  tags = "${merge(map("Name", format("%s", var.name)), var.igw_tags, var.tags)}"
}

# Add default route (0.0.0.0/0) to internet gateway 

resource "aws_route" "public_internet_gateway" {
  count = "${var.create_vpc && length(var.public_subnets) > 0 ? 1 : 0}"

  route_table_id         = "${aws_route_table.public.id}"
  destination_cidr_block = "0.0.0.0/0"
  gateway_id             = "${aws_internet_gateway.this.id}"

  timeouts {
    create = "5m"
  }
}


#####################
# SECOND TERRAFORM
# Spin EC2 to run as NAT

resource "aws_instance" "ec2-nat" {
  count                       = "${var.instance_qtd}"
  ami                         = "${data.aws_ami.nat.id}"
  availability_zone           = "${var.region}a"
  instance_type               = "${var.instance_type}"
  key_name                    = "${var.aws_key_name}"
  vpc_security_group_ids      = ["${var.sg_ec2}","${var.sg_ops}"]
  subnet_id                   = "${var.public_subnet_id}"
  iam_instance_profile        = "${var.iam_instance_profile}"
  associate_public_ip_address = true
  source_dest_check           = false

  tags = {
    Name                          = "ec2-nat-${var.brand}-${var.role}-${count.index}"
    Brand                         = "${var.brand}"
    Role                          = "${var.role}"
    Type                          = "ec2-nat"
  }
}

# Add default route (0.0.0.0/0) to aws_instance.ec2-nat

variable "default_route" {
    default = "0.0.0.0/0"
}

resource "aws_route" "defaultroute_to_ec2-nat" {
  route_table_id              = "${var.private_route_id}"
  destination_cidr_block      = "${var.default_route}"
  instance_id                 = "${element(aws_instance.ec2-nat.*.id, 0)}"
}