The tenant address is showing as the new property address
There isn't a way of changing the tenant address since the Housing Act requires that an AST governs the rental of a property to tenants who will use the property as their primary residence. As such, the best place to contact them in writing or to serve any notices will be the property address.
Before the tenancy begins, there are no notices that can be served that can have any bearing until the tenancy starts, so again there is no need to use any address other than the property's to formally contact the tenants.
In the meantime you should obviously feel free to continue to correspond by email and phone to make arrangements directly with the tenant.