subletting

Tenancy Types and Management

hi all,
looking for advice regards subletting on my property which i have rented to 4 students. their tenancy is up in September, one tenant wants to leave early, and has found a replacement person, who seems acceptable. i am reasonably happy to comply, but not sure whether to allow them to sublet or should i just change over the contract to the new tenant? unsure where i stand legally on this, i used the RLA AST tenancy documents. Any benefit or drawback to subletting or changing contracts directly? There would be 5 months left on tenancy either way. A new set of tenants likely to come in September.
thanks, sean