This is the dilemma for many landlords. Let's look at both answers available for this question.
Yes, change the locks!
For many landlords, it's not so much about trusting the tenant, but about not even needing to trust them. A change of locks puts to rest any lingering doubt that the previous tenant might have made a copy of the key. Even if the previous tenant wouldn't dream of trespass, it's impossible to know how protective of the keys they were during their tenancy. They could have been copied by a third party at any stage during that tenancy - you can never know either way, so changing the locks puts paid to any possible doubt. Keys that are in the custody of a business tenant might be passed to any number of their employees, so you can enver be too careful.
Furthermore, changing locks after every tenant is a selling point to letting agents. You can reassure them that they won't have to deal with any trespass issues for the next tenant because of copied keys. Furthermore, the lock itself could have been damaged by the previous tenant - a new lock takes away that potential problem.
No, don't bother!
The only possible reason for not changing locks is simply cost. If the landlord is in a competitive market, the cost of replacing an entire lock every 6 months (potentially) has to be considered as a negative. However, the landlord need not go to that extent to ensure security. They could consider rekeying the lock. This is a "lighter" alternative to completely replacing the entire lock. Instead, rekeying involves adjusting the pins in the lock mechanism, preventing older keys from gaining access to the property.
Clearly, the only thing stopping a landlord from maximising security to their property is cost. And with rekeying as an option, the cost argument isn't a very strong one. Article kindly provided by locksmithinguildford.co.uk