To change an existing lease, you'll need to make a Lease Amendment document and usually notify the tenant of the change at least 30 days in advance. The amendment should be saved along with the original lease for your records. The most common lease changes include adding a pet provision or adding a roommate. If you need to make major lease changes, it may be best to wait until the end of the term of the lease. If you have questions about changing a lease, ask a lawyer.