When a landlord or tenant creates a Lease Addendum, the original tenancy agreement remains in effect with the amendment included as an attachment.
However, in order for a Lease Addendum to be considered legally binding, all parties must consent to the changes by providing their signatures. One party cannot unilaterally make a change because the original tenancy agreement is a legally binding contract that cannot be altered without both parties' approval.
Therefore, during a fixed-term lease, neither party can create a Lease Addendum and demand the other party to consent to the alterations. For example, if a landlord wants to institute stricter quiet hours halfway through a one-year fixed-term tenancy, the tenant has to agree to the change. If the tenant does not agree, the landlord has to wait until the end of the lease term to try enforcing the amendment.