Imagine – if someone could change recorded video footage and read it back to the server, it would make it hard to use the video footage as evidence as it could be tampered. Even if it is not the same, I believe the design consideration is the same, you cannot change an event after it is registered and stored.
If the scenario you have is that you want to add metadata to an Analytics Event later after some lengthy video analysis you might workaround by adding a new event instead of changing the original event.