Hi Bartek: This is another example of an English idiom (it's understandable in Canada, too. The second meaning you listed is the most accurate. The "sweeps clean" part refers to making many changes.

I just found out today that there can be more than one meaning for this phrase. I only ever heard it used in the second way mentioned above. It is a phrase of warning to anyone with expectations that a new hire is going to be as sharp and efficient as they seem at first. When a broom is new its bristles are fresh and strong . After a broom has been used for a while it sags a bit and doesn’t clean as well. So a new hire is happy to have a job and is on time and hard-working. After a time, the job is routine and dull and the person loses his or her enthusiasm. It doesn’t mean they flunk out, but they lose interest in doing a good job. The other use of the phrase meaning that a new boss is going to make changes doesn’t mean very much. I don’t know why it would have become a proverb if this is all it meant.

To be honest I've never heard of this idiom. It's British you say? It's a new one on me.