This isn't a feature that is currently available. I recommend putting in a feature request. I know I will be as this is something that the users in our instance are really looking for.
This has potential benefits and drawbacks. While this is something we REALLY wanted when initially implemented in order to maintain project status updates, we decided on a custom field for that. This way the PM UPDATE is editable and trackable.
The problem with editing updates is that if someone edits an update further back ont he stream the context is lost for all following posts.