Hmm!! It looks as though Project item does not offer the same control of changes as you get with a WO on a change.
A workround would be to have a custom field on a chnage, say call it "Predecessor finished" and have a DB rule which updates this whenever a chnage completes (using udate related item> successor).
Then have another rule which checks when a change starts that this custom field is set to Yes and sends a warning if not. You would need to ensure this field was set to yes if a chnage was not pasrt of a project.