Thanks, and yes, that part I got - my problem was then how do I trigger the Change node from outside the MQTT->Change->UI-Node chain, without inserting a new msg.payload in the chain.
The only solution I found so far is to store the state of the device in a global or flow variable, and do a Switch + Change node combination that checks the value of the flow variable and then sets the msg.enabled as required, but this seems somewhat counterintuitive and also has the distinct disadvantage that I can't actually trigger it when the value changes, have to rely on the MQTT node to update...
