Joe has a written agreement with Naomi in terms whereof he is renting a house from her. After the lease has been running for a few months, they agree orally that, from now on, Naomi (the lessor) shall pay for a gardening service to maintain the garden. In return, Joe (the lessee) will pay an extra N$ 100 in rent, since he no longer has to take responsibility for looking after the garden. Is this change valid?