I think this was already asked on the forum, it all depends on how MS is blocking the older versions from connecting to the servers. If the client has some sort of handshaking going on with the server and exchanging versions numbers or a "secret" word, then it will be difficult to bypass this. It could be simple also, such as a change to an ini file or a registry setting. It will take someone who has time and willingness to find out.
there is also the alternative chat clients, you could try inplace of the newest MSN client.