下記のように配列で作成したリソースはvariableを変更した際、配列を再作成してしまいます。
これをfor_eachでリファクタリングする際、手作業やterraform state mvで１つづつterraform.tfstateを更新するのですが、moved blockを使用すれば考えることを減らしてリファクタリングできます。

befor

route53_record.tf

data "aws_elb_hosted_zone_id" "main" {}

resource "aws_route53_record" "test_record" {
  count   = length(var.test_recors)
  zone_id = data.aws_route53_zone.selected.zone_id
  name    = element(var.test_recors, count.index)
  type    = "A"
  alias {
    name                   = var.test_elb_dns_names
    zone_id                = data.aws_elb_hosted_zone_id.main.id
    evaluate_target_health = true
  }
}

variables.tf

variable "test_recors" {
  type = list(any)

  default = [
    "test1.xxxx.jp",
    "test2.xxxx.jp",
    "test3.xxxx.jp"
  ]
}
variable "test_elb_dns_names" {
  type = list(any)

  default = [
    "awseb-AWSEB-xxxxxxx-xxxxxxx.ap-northeast-1.elb.amazonaws.com"
  ]
}

after

route53_record.tf

data "aws_elb_hosted_zone_id" "main" {}

moved {
  from = aws_route53_record.test_record[0]
  to   = aws_route53_record.test_newrecord["test1.xxxx.jp"]
}
moved {
  from = aws_route53_record.test_record[1]
  to   = aws_route53_record.test_newrecord["test2.xxxx.jp"]
}
moved {
  from = aws_route53_record.test_record[2]
  to   = aws_route53_record.test_newrecord["test3.xxxx.jp"]
}

resource "aws_route53_record" "test_newrecord" {
  zone_id = data.aws_route53_zone.selected.zone_id

  for_each = var.test_newrecords

  name = each.value.fqdn
  type = "A"

  alias {
    name                   = var.test_elb_dns_names
    zone_id                = data.aws_elb_hosted_zone_id.main.id
    evaluate_target_health = true
  }
}

variables.tf

variable "test_newrecords" {
  type = map(any)

  default = {
    test1 = {
      fqdn = "test1.xxxx.jp"
    }
    test2 = {
      fqdn = "test2.xxxx.jp"
    }
    test3 = {
      fqdn = "test3.xxxx.jp"
    }
  }
}

variable "test_elb_dns_names" {
  type = list(any)

  default = [
    "awseb-AWSEB-xxxxxxx-xxxxxxx.ap-northeast-1.elb.amazonaws.com"
  ]
}

planで変更がされていることを確認し、applyで適用する。

$ terraform plan
  # aws_route53_record.test_record[0] has moved to aws_route53_record.test_newrecord["test1.xxxx.jp"]
.
.
.

terraform.tfstate更新後はmoved blockは不要なので削除します。