How can I modify an auth_token of a redis cluster if the redis engine version is below 5.0.5?

As you pointed out, prior to Redis 5.0.5, changing the AUTH token was not supported. The only way I found to do this is to stand up a replacement cluster, which at that point you should upgrade the Redis version.

References

  • Amazon Elasticache Announces Support for Modifying Redis AUTH
  • Redis Password Change