In previous years I was a landlord and used to change the locks when someone moved out - I had two sets and turned them around after each tenancy. It was fairly easy and I can if I can do it anyone can. There was a vaguely amusing story about a current tenant getting woken up in the middle of the night as someone was trying to open the door and couldn't as the locks had being changed. The current tenant opened the door and there was someone there, drunk and distraught, asking him where was Tommy (invented name of previous tenant) - he had moved out about six months before. It later transpired that the previous tenant had broken up with this boyfriend and the midnight caller was the ex-boyfriend who didn't know that Tommy had moved out.