The first thing you inherit with a prewar apartment is not the crown molding. It is every key that was ever cut for that door and never handed back. Former tenants, their dog walkers, a contractor from a renovation two owners ago: you have no way to count them.
Rekey before you unpack
Rekeying swaps the pins inside your existing lock so a new key operates it and the old keys no longer match the pinning. The hardware stays; the key population resets to the ones in your hand. It is the single most useful thing a new tenant or owner can do in week one.
Get to know your mortise lock
Most prewar doors carry a mortise lock: a rectangular steel case set into the door edge, running both the latch and the deadbolt through one body. Two things follow. First, these locks are rebuildable, and a good one is worth servicing rather than replacing. Second, modern drugstore deadbolts do not drop into a mortise prep, so upgrades need someone who knows the format.
A prewar mortise lock is more like a watch than a padlock: built to be serviced, not thrown away.
Count doors, not just the front one
Service entrances, cellar access, the door from the garden, the one behind the gate under the stoop. Buildings collect doors over a century. Walk them all and ask which ones share a key, because a rekey that skips a door is an unfinished rekey.
Questions new neighbors ask
Does the landlord need to approve a rekey?
In most rentals the lease speaks to lock changes, and buildings often require that management hold a copy of the key. Rekeying with a copy provided usually satisfies both sides, but read your lease first.
Is a shiny new lock better than my old mortise?
Not automatically. A sound mortise body with a fresh cylinder is excellent hardware. Replace when the mechanism is worn, not because of its birthday.