Also, I'll post relevant parts of the lease when I get back to my computer.
I'd send them a certified letter with a copy of your current valid lease. Explain that you do not agree with the addendum and you intend to keep paying your rent, occupying the property under the terms of the original lease, and that they can either:
A: Honor the original lease, as signed.
B: Break the current lease with no penalty and a full refund of your deposit.