Changing the lock probably violates your lease. And if you wanted to catch them entering your apartment without notice, something like a camera that creates readily identified evidence would be a better idea.
I wasn't sure if it can be configured to prohibit key access. How strong is it? If it can, it's unauthorized. If it can't, it's not keeping your landlord out.
He said he wanted to know when the landlord came, not keep him out. As far as I know, it won't even attempt to stop the key from opening the lock, but it will detect it.
