Can landlord change lease term from annual to month2month?

[Qiu Jennifer]

Hi all, 

I have a tenant that signed a 18 months lease and kept delaying on their rent payment in the first 6 months. We have a clause in the original contract saying that if they cannot get caught up on payment by the end of the 6th month, the lease agreement will be terminated. Now walking towards the end of the 6th month and they haven't paid in full, can I terminate the contract by giving them 30 or 60 days to move out by the end of the month？If they do not leave by the move out deadline, can I start eviction process? Or as plan B, can I change the current lease to a month to month lease? They are still paying but very late each month, so I'm afraid of them becoming no-pay tenants down the road. Any comments would be greatly appreciated!