A lease is a contract between landlord and tenant.  A valid contract requires an offer and acceptance.  Since you had not signed the new lease with the terms of the first renewal letter, no contract existed.  Therefore, the offer (terms in first renewal letter) could be revoked prior to your acceptance.  Since there wasn't any existing contract (new lease on the terms of the first renewal letter), it does not appear that you would have any grounds for enforcing the terms of the first renewal letter.  If you had signed a new lease based on the terms of the first renewal letter, you could argue that the terms of the first renewal letter were binding and you and the landlord would be subject to its terms.