When a new tenant moves in whose responsibility is it to change the locks? The new tenant or the landlord?

It is the landlord. However, no law requires it. Usually the present locks are just re-keyed, there is no reason to actually change the lock itself.

Each State Regulates it's own Rental Laws, so it depends. But in MA the tenant is responsible for the cost of the locks. The Landlord can charge a "lock fee" for re-keying or replacing the lockset if that's his policy.

There will be people that say the tenant must give the landlord a copy of the key. Again, it depends on the state. In Ma, the landlord does not have a right to unfettered access (have a key) because, with the exception of an emergency, the landlord must give no less than 24 hours notice and the tenant has the right to be there when any work is done.

This is the PDF of Alaska Tenant and Landlord Rights:

"Tenants can insist that the landlord maintain or replace the locks if the residence is not secure.51

Tenants may want to add an extra lock on their own to increase security. With the landlord’s permission,

a tenant may add locks that can be used from the inside, such as chain bolts. If the tenant makes holes in the door or frame, he or she must leave the lock in place when moving out.

Neither a landlord nor a tenant may be locked out. If a landlord adds or changes locks, new keys must be given to the tenant right away.

Before changing locks, the tenant must generally get the landlord’s written permission. However, in an emergency, when the landlord can’t be contacted first, the tenant can change locks, provided he or she

gives a new set of keys to the landlord within five days" http://courts.alaska.gov/forms/pub-30.pdf

SO in a nutshell, you would need to speak with your landlord. If the landlord allows you to change the locks, since that is per your request you would probably be responsible for paying for the new locks. When I got new locks on my doors when the landlord allowed it because I had concerns about safety and he had just purchased the townhomes and didn't know the history of who had rented, he allowed me to pay a locksmith to get new locks. Then I gave him copies of the keys.

No law indicates directly that locks must be changed. In the end, it all really depends on what is agreed upon in the contract.

The landlord is responsible