Yearly lease change to monthly lease

There's no such thing as a monthly lease. With no lease, either tenant or landlord can terminate with 30-day notice, and landlord can raise rent at any time, with you either liking it or lumping it.

http://www.housingnyc.com/html/resources/attygenguide.html#3

He's probably trying to get the lease onto a pricier peak-season schedule.

Appreciate it! But I wonder why they have to raise the rental, given that winter is hard to find a tenant for a landlord. I'd rather move out in winter to find a better deal than wait till summer to be kicked out. Right?

If your lease is up soon you can always move out then, if you find a better apartment/deal.
People move year-round and the landlord will rent the apartment to another renter, even in winter.
If you move out before your lease is up your landlord keeps your security money.
Does your lease end next summer? Start looking in the Spring for a new apartment to rent. You can't "be kicked-out" until your lease is up. As NWT points out above, your landlord can't pull a switcheroo to a "monthly lease".

The lease has to be up, or else they couldn't raise the rent. Find another apartment.

I agree with aboutready's legal interpretation of the facts.

Is that normal to stay for a few months without a lease? It seems I need just 2 months stay after my lease is up in Dec.