Generally speaking, tenancy agreements, once signed, are binding. It’s not necessarily straightforward to change a tenancy agreement, such as would be required if the landlord is interested in adding or removing a person or persons to the terms of the agreement. There’s no way for one party to force these changes through while still remaining compliant with the relevant regulations specified in UK law. Rather, any changes to the tenancy agreement must be mutually agreed upon by both the lessor and the lessee.
Either a new agreement can be drawn up, or the original on can be amended. Be careful, though — verbal agreements on their own aren’t binding, although they can be reinforced by written evidence. One example of a way a verbal agreement could be ratified is by both parties agreeing to a change in the amount of money to be paid in terms of rent. Because there will be receipts and invoices to create a paper trail, the verbal agreement can be proven if need be.