Changing letting agent when buying an existing let property

Letting Agents

We are buying an existing tenanted multi-block unit (not HMO) in England and have received a copy of the existing tenant agreements with the current lettings agent. We are thinking about changing the existing letting agents when we sign contracts.
What happens to the signed agreement between the old landlords and the letting agency? Do we buy this current landlord contract and hence letting agreement with the tenants? Are we tied into the existing letting agent for a notice period? We cannot see anything within the tenancy agreement with each tenant, so where would this be if available at all?