Payload Enricher with request-payload-expression="payload.username"

While trying out the enricher example by using Java DSL, I am unable to replace the payload that contains User object with only username field using the method:
.enrich(p -> p.requestPayloadExpression("payload.username"))
--> https://docs.spring.io/spring-integration/reference/html/content-enrichment.html#payload-enricher

However, the payload functionality works when I use the transform method that takes in an expression parameter
.transform("payload.username")

Is the transform method the only way to update the payload?

Start with the Java DSL calls shown in the issue: enrich with requestPayloadExpression("payload.username") and transform("payload.username"). Read the linked payload-enricher documentation and compare the two expression entry points using the enricher example. Done means determining whether the enricher is expected to replace the payload and documenting or reproducing the correct behavior.