Can a landlord change the terms of my lease without my signature?

I signed a year lease, then renewed with a rental increase. At the end of that lease, I requested 2 extra months & the landlord agreed. She simply crossed out the end date and wrote the new end date on my last signed lease. She expressly said rent was the same as my lease. I moved out just as the complex was sold to another firm. After 41 days I had not received my security deposit. I had cleaned, vacuumed and washed windows before leaving. The place was cleaner than when I moved in. Upon calling for my security deposit, the new landlords told me 3 times someone would call me but no one did. I finally spoke to someone today who says they are keeping my deposit because 1) they had to clean, 2) my record indicates late rental payments (this is not true. I paid each month ON THE FIRST AS DIRECTED BY MY LEASE) 3) Non-payment of rent. They claim rent went up for the month to month lease and I hadn''t paid enough. I signed nothing. I would have moved home had I known.