Rekeying
Who has keys to your door?
A former tenant?
A former employee?
Someone you loaned your keys to?
Who else?
Rekeying a lock involves changing the pins inside the lock so that the old key doesn't work and a new key will work. It's the cheapest security you can buy if you are unsure who has still has keys to your locks.
Curt’s Lock & Key Service, Inc.1102 Main Ave
Fargo, ND 58103-1755