Kafka Streams upgrade.from config while upgrading to 2.1.0

Hi,
I am in the process of upgrading my Kafka streams services from 1.1 to
2.1.0. I am following the upgrade guide:
https://kafka.apache.org/20/documentation/streams/upgrade-guide .

My service is running on kafka version 2.0 and using kafka streams 1.1.1. I
updated my kafka-streams to 2.1.0 but DID NOT pass the config value
`upgrade.from` (it is null), as can be verified from the logs
```
[INFO ] 2019-03-26 18:05:49,550 [main]
org.apache.kafka.streams.StreamsConfig:logAll: StreamsConfig values:
application.id = application_id
application.server =
bootstrap.servers = [bootstrap-server-01:6667,
bootstrap-server-02:6667]
buffered.records.per.partition = 10000
cache.max.bytes.buffering = 10485760
client.id =
commit.interval.ms = 15000
connections.max.idle.ms = 540000
default.deserialization.exception.handler = class
org.apache.kafka.streams.errors.LogAndFailExceptionHandler
default.key.serde = class
org.apache.kafka.common.serialization.Serdes$ByteArraySerde
default.production.exception.handler = class
org.apache.kafka.streams.errors.DefaultProductionExceptionHandler
default.timestamp.extractor = class
ziggurat.timestamp_transformer.IngestionTimeExtractor
default.value.serde = class
org.apache.kafka.common.serialization.Serdes$ByteArraySerde
max.task.idle.ms = 0
metadata.max.age.ms = 300000
metric.reporters = []
metrics.num.samples = 2
metrics.recording.level = INFO
metrics.sample.window.ms = 30000
num.standby.replicas = 0
num.stream.threads = 3
partition.grouper = class
org.apache.kafka.streams.processor.DefaultPartitionGrouper
poll.ms = 100
processing.guarantee = at_least_once
receive.buffer.bytes = 32768
reconnect.backoff.max.ms = 1000
reconnect.backoff.ms = 50
replication.factor = 1
request.timeout.ms = 40000
retries = 0
retry.backoff.ms = 100
rocksdb.config.setter = null
security.protocol = PLAINTEXT
send.buffer.bytes = 131072
state.cleanup.delay.ms = 600000
state.dir = /tmp/kafka-streams
topology.optimization = none
upgrade.from = null
windowstore.changelog.additional.retention.ms = 86400000
```

The application is consuming messages as expected and is not failing.

I even went through the steps mentioned in the upgrade guide and did 2
rolling bounces with the correct config in place. Then I rolled back my
application to Kafka streams 1.1.1 and it was running as expected.

I went through KIP-268
<https://cwiki.apache.org/confluence/display/KAFKA/KIP-268%3A+Simplify+Kafka+Streams+Rebalance+Metadata+Upgrade>
and
it mentions `Kafka Streams need to be configured with
upgrade.from="<old.version>"for startup`, but the service runs fine even if
I don't configure it.
It also mentions `user prepares a second round of rebalance; this time, the
configuration parameter upgrade.from must be removed for new startup` but
the application runs as expected even if I don't remove the config
parameter.

So, my question is, what is the significance of the variable `upgrade.from`
as it does not seem to have any effect on our kafka streams service.

Regards,
Anirudh