changing from an agents to your Tenancy agreement

Tenancy Types and Management

situation -tenants are currently on an 12month tenancy with no statutory periodic tenancy written in this was done through an agent and expires on 8th Aug I did the inventory and secured the deposit they are going to stay and transfer on to your AST they pay 3 months in advance and will be changing to i month in advance

Q - Not sure how to deal with this ie can I back date to cover the whole time they have been there ( I appreciate I will be running on the statutory periodic tenancy) Or do I just start it form the 8th if so -will the original inventory still count(not practical to complete a new one) are there any other considerations ie deposits