In order to improve connections for their users, Yahoo! switched server locations yesterday, from California to the East Coast.
About 50% of Yahoo! .com users are actually outside of the US, and so a time of 8PM EST was chosen so that most users would not notice the few second disconnect that would take place.
Their servers have totally new IP and hostnames, Messenger receives the servers on the fly, and can choose one, and so it has no problems connecting. Yahoo! Chat clients use hard coded hostnames, that point to the old servers that have been replaced. Connecting to these servers puts you into what appears as a parallel Yahoo! Chat!
The protocol remains as before. There has been no upgrade for bootcodes, bots etc…