A landlord cannot change an existing lease agreement without the tenant’s consent. After all, a lease is a contract with terms that both parties have agreed to: the landlord making material changes to the terms of an existing lease without the tenant’s consent would be unfair and potentially burden the tenant with unforeseen costs or responsibilities.
This is true even for the smallest changes - so if your landlord tries to change the late rent fee, add additional fees for things like key replacements or amenities, or impose new behavior rules then you should tell them point blank that it's illegal.
This is not to say that your landlord cannot change the lease agreement at all. If your landlord wants to alter your lease agreement while it is still effective they can ask for your consent to the proposed changes. They'd do this using a document that lawyers call a lease addendum. If you both consent to the changes, they become effective as a part of your lease agreement. If you don’t consent then they can’t change the lease
What your landlord can do is make changes to future lease agreements. Any new tenant would then be subject to the new terms, and when your lease expires you are subject to the new terms if you decide to renew.