If it is a rental house he has the right to charge you for any changes you make to the home while you are living in it. Look at your rental agreement and see what it says about changes being made to the home. If you make a change that can be undone when you leave it should not be a problem but changing the locks is a something that should have been done before you moved in to ensure that the landlord has the current key because after all it is his house.

 I should have clarified more... We just moved after renting the house for 18mos. And are now waiting for our deposit to be returned. One of the stipulations our landlord mentioned was that if we did not return all keys, then the cost of re-keying the house would be deducted from deposit. We are missing 1 key. I don't agree with this though, because #1, let's say i do return both keys... Who's to say i haven't already made and distributed 5 copies? And #2, we learned shortly after we moved in that he had given keys to 3 neighbors... Therefore how can he fairly hold me responsible for access he's given to others (and unbeknownst to me!- that was not indicated in our agrmt). Thanks in advance for any replies!!