First, the lease language controls, so both LL and tenant need to follow it strictly.
If it says nothing about the locks, then tenant cannot change them without permission. If locks are changed, LL needs to get copy of keys for several reasons: to gain access in the future (with limitations spelled out by law and in the lease), and in the event tenant loses keys or locks himself out.
Also, if tenant is given permission to change locks, LL must demand ALL the parts to the original locks. The tenant may put on a cheapo lock which could be replaced by the original lock when they leave.
