Even if they give a new key to the landlord/agency?
If you don't know when the locks were last changed and have no idea who would own keys surely it makes sense to change the locks?! But apparently this counts as a breach of tenancy...I'm pretty shocked on discovering this.
YABU - tenants shouldn't be allowed to change locks even if they give spare keys to the landlord
YANBU - It's bonkers that tenants are expected to leave financially or emotionally important possessions in a potentially insecure property