Moving Letting Agents

Tenancy Types and Management

Hello,
I am new to lettings and will be managing some properties for other people as an agent, a few properties are on periodic contracts with other agents. We hope to have a smooth transition with the other agent but my question regards their tenancy agreements, the other agent found the tenants and arranged the agreements, will they have to sign new tenancy agreements and if so do they have to be ASTs or can they be straight to periodic as technically they are the same tenants with the same landlord only the agent has changed.
thank you