This should work when you use the mAirListDB: import the news file into mAirListDB and note its ID. Then add a “specific item” entry on top of the hour template and enter the ID.
Very important: Remove all cue markers from the news item! Otherwise it might be cut off when it’s longer than the initial version.
You may also want to check the “don’t prebuffer” option in the properties dialog of the news item to make sure that the (old version of the) item isn’t prebuffered by the time the new version is downloaded.