My policy has been to require a lease up front and then let it revert to month-t0-month thereafter. That way you have them for a set amount of time to cover your rent-up expense, and you both have the flexibility to terminate afterwards.
If you demand a new lease in order to stay, what if they were planning a move in 3 or 6 months and you are forcing them to move now instead? If they are good tenants, better for you to keep them on for a few months longer.
