When I take possession of my new apartment after the closing, can I change the locks to protect against the former owner retaining access to the unit and mailbox?

Generally, at the closing, the seller hands over ALL sets of keys to the buyer and should not retain any access to the apartment, mailbox, storage unit (if applicable), building (if non-doorman) and any other applicable entry points.  Naturally, ownership is being transferred and therefore, the seller should no longer have any rights to access these spaces (by key or otherwise).

However, as the buyer, if you are concerned that the former owner may keep a set of keys, in the interest of security, you can opt to change your locks under certain circumstances.  First, you should review the bylaws / house rules for your building to determine if there are any specific rules and procedures for changing locks.  Second, speak with your building’s super about doing so, before you do anything, as he/she is usually the go-to person for all things relating to changes to the building (and can issue basic approval, in some cases).

For other areas requiring keys, such as the mailbox and storage space (if you own one), it may be more difficult to change those locks, as they could be standard for the building.  Regardless, the super should be able to provide you with more information and potential options, if any are available.