The simple answer is no. The landlord cannot change the terms of your lease without your consent. The landlord could have demanded that you pay increased rent in order to extend the term of your lease, but did not do so. The landlord's action of simply changing the date on your lease indicates an agreement that the terms of your lease would remain the same during the extended period of your tenancy. Thus, assuming that your landlord does not dispute the events as you recite them, you should be able to get your security deposit back. In the real world, however, it is likely that the landlord will dispute your account, and a court will have to decide which of you is telling the truth.