Rename kong API key name

Question

We want to rename kong apikey name from apikey to api_key, for that we tried

curl -X POST http://localhost:8001/plugins/ \
    --data "name=request-transformer"  \
    --data "config.rename.headers=apikey:api_key"

as described here
We get

{"code":5,"fields":{"consumer":null,"service":null,"route":null,"name":"request-transformer"},"message":"UNIQUE violation detected on '{consumer=null,name="request-transformer",route=null,service=null}'","name":"unique constraint violation"}

Then,request is updated as

curl -X POST http://localhost:8001/plugins \
    --data "name=request-transformer"  \
    --data "name=ourservice" \
    --data "config.rename.headers=apikey:api_key"

result :

{"code":2,"fields":{"name":"expected a string"},"message":"schema violation (name: expected a string)","name":"schema violation"}

We use docker based kong API GW (kong 2.7.1)