I would send the notice, then go month-to-month. Changing the pay date to the middle of the month is a bad idea (been there, done that), unless you are also careful to move the rental period begin/end to the middle of the month. Otherwise, they are into you for two weeks of free rent before the rent is even due. Then if they are a couple weeks late, it is suddenly the next month. Now they are into you for a free month's rent, but are only two weeks late...it's downhill from there.
In my experience, tenants are like children: they behave best with firm boundaries and discipline.