I think they just went ahead and did it. The change prevented MSN users from connecting with the AOL service via the new MSN Messenger. MSN Messenger is the company's first entry into an already popular category of messaging services. Yahoo, AOL, and Excite all offer instant messaging within their spate of Internet mail and chat services. Instant messaging amounts to a kind of live email.
Users can summon each other instantly and exchange messages in real time.
It's similar to open chat rooms on the Net, but is specific to two or more acquainted users. Until recently, however, users have had to belong to the same service to send each other instant messages. Citing the convenience of interoperability among services, Microsoft built MSN Messenger so that users could access AOL Instant Messenger accounts. Yahoo on Wednesday released beta software that added AIM compatibility. But following the release of MSN's new service Thursday, AOL cried foul, claiming that existing AIM users had to switch to the new Microsoft service in order to achieve the cross-service capability.