MQTT message transformation?

I have an auto discovered MQTT entity:


{
  "~": "home/light/livingroom",
  "name": "livingroom",
  "unique_id": "MQTT-livingroom",
  "cmd_t": "~/set",
  "stat_t": "~/state"
}

I am expecting basic/raw message (no-json). Unfortunately the HA MQTT integration always expects capitalized verbs, i.e. ON/OFF and also sends these via set.

Checking home/light/livingroom/state I see the state is on. HA never displays the on state since it expects ON.

How to I transform these verbs to lowercase so HA understands these?