What is a lease?
A lease as static document. It’s simply a contract between a landlord and a tenant that states all the 'tenancy terms' that each party has agreed to like rent amount, the fixed period (contract period), number of occupants, pets, special conditions etc.
Although during a tenancy things can change the static lease agreement itself does not need to change. Rather an additional agreement between the landlord and tenant should be made that update the 'tenancy terms'.
For example, if the tenant wants to get a pet dog and the landlord agrees as long as the tenant gets professional flee treatment at the end of the tenancy, a separate agreement is made in writing. This agreement updates the 'tenancy terms'.
For more clarity, things don't always have to be agreed on during a tenancy for the 'tenancy terms' to be updated. For example, the landlord can increase the rent as long as 60 days notice is given and it takes effect outside the fixed term period (after the lease end date). Once the increase takes effect the new rent amount forms part of the 'tenancy terms'.