I often take notes in the Timeline in anticipation of my next conversation with a customer. Most often I will do that as a "meeting" but sometimes I start as an update and that converts to a meeting. I would love the ability to change the the type of message from one type to another.
Each activity type can have a very different form layout and that is why we have restricted the ability to change the activity type. But I also understand your use-case, few other customers have also pointed out the same thing. We will look into the possibilities of enabling this feature.
Sometimes the type gets set mistakingly (accidentally hit meeting instead of call, etc) and it would be great to be able to change it to the correct type without having to copy and paste everything into a new activity.