Change fron lodger status to tenants

Tenancy Deposits

We are due to move out of our 4 bed property in September to re-locate. We currently have three lodgers and our vacating will enable a fourth sharer. I would like to know if we will have to upgrade everyone from lodger to tenant status and issue ASTs? Also, we have so far made a deposit arrangement that allows the lodgers to have their last month free, subject to any deductions for damages according to the inventory. From September are we legally required to put deposits in an official scheme?