I have been renting an apartment with a friend on 6 month leases where we have been co-signees to the lease. We have been in the same apartment for over 2 years. With 2 months left on the lease my roommate went to the apartment manager without my knowledge or permission and asked that his name be removed from the lease. He is fully capable of paying for his half of the rent. The apartment manager agreed to his request, he moved out taking his personal items, and then the apartment manager informed me that I was now responsible to pay the lease. According to the manager, my roommate provided the apartment with a notarized letter that they accepted and which was the basis for them removing his name from the lease. I have asked for a copy of this document, and to date they have refused to provide it

I have told the manager that I did not agree to this arrangement and have now told the manager that I intend to pay 1/2 the rent to them for the last two months, since this is what my obligation has been all along. I feel the apartment manager/owner has burdened me unjustly and that they should never have allowed my roommate to have his name removed from the lease without my approval or consent.
As I said he is more than able to pay his half.

Please advise if you think my position is defensible? correct? or if I will end up being messed over more than I already am?

Nope, you have to pay full rent to the larndlord, you can them sue the other tenant for his 1/2 of the rent.

Also in fact the apartment management can not legally change the lease unless all parties agree to it.

Your only defense is that in their changing the lease, they broke it and now there is no valid lease but you are on a month to month, and could use that as grounds to move out.

But the last thing you can do is merely pay 1/2 or the rent

I have written a letter to the manager advising them that they have seriously damaged my rights under the lease agreement that was jointly signed by the myself, my roommate, and the Apartment. I have told them that they made no effort to advise me that they were altering the lease and that this is either evidence of a failure of their contractual obligations or gross negligence.

So, I guess your point is that they altered the lease without my knowledge or agreement, and that by doing this they invalidated the lease.

I am offering them 1/2 rent for the remaining 2 months. The total rent is a little over 700, so this is not that much to them, but it is a big deal to me.

I have also told them that I will not be signing a new lease or a lease extension and that I will be out of the apartment no later than the end of Feb.

If they don't accept my proposal, I guess my next move is to advise them that their unilateral action has invalidated the lease agreement and as a result of their breaking the lease I will be moving out as soon as I can find altenative living arrangements.

Thoughts? Do you think they will sue me for 700 dollars.

First you need to read the lease. Most joint lease arrangements provide that each roommate is responsible for the full rent.

You have a case against the roommate, you may have a case against the landlord for allowing the roomie out of the lease early without your knowledge.

Before you tell the landlord they violated the lease, you NEED to read it carefully to make sure they actually have voided it.

Thank you. You've been a big help.