The month to month lease you have, can not be unilaterally extended by you.
If the landlord accepts six months in advance, he appears to waive his right to have a one month lease, and you should be able to stay for the length of the prepayment.
Having the ability to prepay six months sounds like an attractive incentive for a landlord, and should be presented to the landlord, along with your promise to pay in advance when that 6 month term is completed, as incentive for entering into a 1 year lease at the present time.
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